The 3rd Marine Division's baptism of fire came in the steaming jungle interior of Bougainville, the largest island in the Solomon Islands chain. Bougainville in November 1943 was a critical objective in the Allied strategy of bypassing and isolating the massive Japanese base at Rabaul while establishing new American airfields to project power northward. The operation, code-named Operation Cherryblossom, was part of the broader strategic framework of Operation Cartwheel — General MacArthur's and Admiral Halsey's coordinated offensive up the Solomons–New Guinea axis.

The plan called for the III Amphibious Corps, which included the 3rd Marine Division, to land at Cape Torokina on the southwestern coast of Bougainville — not the most defensible of beaches, but selected precisely because it was lightly defended. The Japanese had not concentrated their strength there, instead deploying the bulk of their forces at the island's northern and southern tips. General Turnage and his staff understood that speed was essential: secure a beachhead, build airfields, and begin operating from them before the Japanese could mount a serious counterattack.

On the morning of November 1, 1943, waves of Higgins boats and LCVPs ground ashore at Cape Torokina against fire from Japanese defenders who had positioned themselves in a series of log-and-earthwork bunkers covering the beach exits. The 9th Marines landed on the left flank, while the 3rd Marines drove inland on the right. Almost immediately, the men encountered the realities of jungle warfare — ground so saturated with tropical rain that every step sank into mud, undergrowth so dense that a squad could pass within twenty feet of an enemy patrol without seeing them, and the constant threat of Japanese infiltration at night.

The fighting in the days following the initial landing was savage and intimate. Japanese defenders used the terrain masterfully, constructing mutually supporting bunker complexes that covered every likely avenue of approach. Marine rifle squads had to reduce these positions one by one, often using grenades, bayonets, and individual courage to close the final distance when firepower alone could not dislodge a determined defender. Private First Class Henry Gurke of the 3rd Marines exemplified this spirit when, on November 9, 1943, he threw himself on an enemy grenade to save the lives of two fellow Marines, earning a posthumous Medal of Honor. Two days earlier, Sergeant Herbert J. Thomas, also of the 3rd Marines, had performed a similar selfless act, giving his life to protect his comrades from a grenade blast.

By mid-November, the Marines had secured enough ground around the beachhead to begin construction of the Torokina airfield complex. Engineer battalions from the 19th Marines worked around the clock in the mud, using heavy equipment dropped ashore across the difficult beach to grade runways and build the infrastructure needed to support air operations. The Japanese responded by launching a series of infantry counterattacks, none of which succeeded in penetrating the Marine perimeter, and a significant naval counterattack in Empress Augusta Bay, which was repelled by U.S. naval forces in a sharp surface engagement.

The 17th Japanese Army, commanded by Lieutenant General Haruyoshi Hyakutake, launched its most serious counterattack against the Marine perimeter in March 1944, by which time much of the 3rd Marine Division had been relieved by Army units from the Americal Division. The March 1944 offensive struck the defensive positions with three converging columns and achieved initial surprise, but the disciplined American response — combining accurate artillery fire from the 12th Marines with determined infantry resistance — shattered the Japanese assault and inflicted catastrophic casualties on the attackers. The failure of this counterattack effectively ended serious Japanese offensive operations on Bougainville.

The Bougainville campaign demonstrated both the strengths and the growing pains of the 3rd Marine Division as a combat organization. Unit coordination between infantry and artillery improved steadily as the operation progressed. The Marines learned the tactical vocabulary of the Pacific jungle — the value of tight perimeters at night, the critical importance of holding all terrain features that could serve as Japanese observation posts, and the necessity of maintaining aggressive patrolling to deny the enemy intelligence and initiative. These hard-won lessons would be carried forward and refined in the division's next two campaigns.

Of all the campaigns fought by the 3rd Marine Division in World War II, none carried more emotional and symbolic weight than the liberation of Guam. Guam was American territory — the western Pacific outpost that the United States had held since 1898 — and it had fallen to Japanese invasion just days after Pearl Harbor in December 1941. For more than two and a half years, the island's Chamorro people and the small contingent of Americans who had not escaped had lived under a brutal Japanese military occupation. The liberation of Guam was therefore not merely a military objective but a redemptive act, the restoration of an American community that had been torn away by force.

Operation Stevedore was one of the most complex amphibious operations yet attempted in the Pacific War. The assault plan called for a two-pronged landing on the western coast of Guam: the 3rd Marine Division, reinforced, would land at Asan on the northern beaches, while the 1st Provisional Marine Brigade — reinforced with Army units — would land at Agat to the south. Together, the two landing forces would drive inland, link up, and squeeze the island's Japanese garrison between converging attacks. The garrison, commanded by Lieutenant General Takeshi Takashina and totaling approximately 18,500 men, was dug in deeply across the island's mountainous interior, prepared for a prolonged defense.

The landing on W-Day, July 21, 1944, was preceded by an intensive naval and air bombardment campaign, but as the Marines would discover, no amount of preliminary fire could destroy all the prepared positions that the Japanese had spent months constructing. When the first waves of LVTs (Landing Vehicle, Tracked) ground ashore at the Asan beachhead shortly after 0830, they were met by intense fire from defenders who had survived the bombardment in their deep bunkers and caves. The 3rd Marines and 21st Marines went ashore in the first waves, with the 9th Marines following. The fighting at the water's edge was fierce, with Japanese automatic weapons and mortar fire raking the crowded beaches as the Marines struggled to get off the sand and drive inland.

Corporal Louis H. Wilson Jr., commanding a rifle platoon of the 9th Marines, distinguished himself on the night of July 25–26, 1944, in an action that would earn him the Medal of Honor and begin one of the most remarkable careers in Marine Corps history. Wilson and his platoon were assigned to defend a critical position when the Japanese launched a powerful nighttime counterattack. Over the course of several hours of desperate close-range fighting, Wilson repeatedly exposed himself to direct fire to encourage his men, personally killed numerous Japanese soldiers, and refused evacuation despite being wounded three times. His position held. The Japanese attack was repulsed.

Private First Class Luther Skaggs Jr. of the 3rd Marines performed another act of extraordinary valor on July 21–22, 1944, the opening hours of the Guam operation. Serving as a squad leader under intense fire on the newly-won beachhead, Skaggs maintained the position of his unit against repeated Japanese counterattacks, and when the squad's automatic weapons position was threatened by the enemy, he placed himself between his men and the incoming fire, continuing to direct the defense despite severe wounds. His actions on that first desperate day ashore on Guam earned him the Medal of Honor.

The battle for Guam's interior developed into a grinding attritional struggle across the island's mountainous terrain, jungled ridges, and network of Japanese-built fortifications. The Marines fought for Orote Peninsula — home of the pre-war Marine Corps barracks — with particular determination. When the 1st Provisional Marine Brigade retook the old barracks area in late July, the moment was deeply symbolic: a wrong was being righted, and the Marines felt it viscerally. The raising of the American flag over the ruins of the pre-war barracks was an emotional moment for every Marine who witnessed it.

The 3rd Marine Division's zone of attack ran through the center of the island, pushing north and northeast through some of the most challenging terrain on Guam. The Japanese defensive commander, General Takashina, was killed in the fighting on July 28, 1944, and command passed to Lieutenant General Hideyoshi Obata, but Japanese resistance remained fanatical. Individual Japanese soldiers and small units continued to resist in the island's densely jungled interior for years after the organized defense collapsed — one Japanese soldier, Hiroo Onoda, was not even discovered on Lubang in the Philippines until 1974, though Guam's most famous holdout, Shoichi Yokoi, emerged from the jungle in 1972, a tribute to the depth of Japanese military conditioning.

Organized resistance on Guam officially ended on August 10, 1944, twenty days after the initial landing. The cost had been steep: the 3rd Marine Division alone suffered more than 1,700 killed and over 5,000 wounded. Japanese dead numbered in the thousands, with only a handful taken prisoner. The island's liberation was celebrated across the United States as a major victory and as the redemption of an American territory seized by force. The Chamorro people of Guam, who had endured years of occupation, greeted the Marines as liberators with a genuine joy that deeply moved even the most hardened veterans.

Private First Class Frank P. Witek of the 9th Marines earned a posthumous Medal of Honor on August 3, 1944, during the mopping-up phase of the Guam campaign. Witek single-handedly attacked and destroyed a Japanese machine gun position that was holding up his company's advance, then continued attacking enemy positions until he was killed. His valor on that August morning, freely given in the final days of a campaign already won, speaks to the character of the Marines of the "Fighting Third."

No single battle in the history of the United States Marine Corps occupies a larger place in American military memory than the assault on Iwo Jima. When the V Amphibious Corps went ashore on that volcanic eight-square-mile island on February 19, 1945, it entered what would become a thirty-six-day nightmare of subterranean Japanese defenses, volcanic ash that made every yard of ground hard-won, and casualties on a scale that shocked even the battle-hardened Marine Corps. The 3rd Marine Division was a central participant in this ordeal, committed to battle from the division reserve after the initial assaulting divisions became badly depleted, and it fought in the grinding, exhausting operations in the island's interior that ultimately broke Japanese resistance.

Iwo Jima's strategic importance was immense. Located roughly 750 miles south of Tokyo, the island sat astride the B-29 Superfortress bombing routes from the Mariana Islands to the Japanese home islands. Japanese fighter aircraft based on Iwo Jima could intercept and harass those bombing missions. American possession of Iwo would provide an emergency landing field for crippled B-29s — a fact that would eventually save the lives of an estimated 24,000 American airmen — and a staging base for the anticipated invasion of Japan itself. Admiral Nimitz and the Joint Chiefs understood the island had to be taken, regardless of the cost.

The Japanese garrison, commanded by Lieutenant General Tadamichi Kuribayashi, numbered approximately 21,000 men and had spent months preparing the most sophisticated defensive network yet encountered by American forces in the Pacific. Kuribayashi had abandoned the usual Japanese practice of defending the beaches in favor of a defense-in-depth built around an extraordinarily complex system of underground tunnels, bunkers, caves, and interconnected firing positions. Miles of tunnels connected strongpoints across the island, allowing Japanese soldiers to move, resupply, and reinforce under cover, largely immune to the surface bombardment that had softened previous Japanese island defenses. The island itself was Kuribayashi's primary weapon: every rise, every cave, every lava fold was incorporated into the defensive scheme.

When the assault landed on D-Day, February 19, 1945, the first waves of the 4th and 5th Marine Divisions came ashore on the southeastern beaches beneath the brooding mass of Mount Suribachi. The black volcanic sand immediately created serious problems — vehicles, landing craft, and men sank into it, and the steep beach terraces created a physical barrier that slowed the movement inland. Almost immediately, the Japanese opened a devastating fire from the heights and from the fortifications honeycombing the central plateau. Casualties were severe within hours of landing. By nightfall of D-Day, the Marines had secured a tenuous perimeter, but at frightful cost.

The 3rd Marine Division went ashore beginning on D+4, February 23, 1945, committed from corps reserve to relieve and reinforce the battered assault divisions in the center of the island. General Erskine's men took up positions in the middle of the island's volcanic interior, tasked with driving north through the most heavily fortified terrain on Iwo Jima — the belt of mutually supporting pillboxes, bunker complexes, and interconnected cave systems that Kuribayashi had constructed to make every yard of progress cost the maximum possible in American lives.

On the same morning the 3rd Marine Division began its assault inland, the iconic flag-raising atop Mount Suribachi took place — one of the most famous photographs in American history. But for the Marines of the "Fighting Third" pushing north into Kuribayashi's main defensive belt, the flag meant little immediate practical relief. The fighting in their sector was some of the most savage of the entire battle, conducted in terrain that offered no cover, no shade, no relief from the choking sulfurous fumes that rose from vents in the volcanic ground, and no respite from the Japanese fire that poured from positions invisible until the Marines were on top of them.

It was in this environment, on February 23, 1945, that Corporal Hershel W. "Woody" Williams of the 21st Marines performed the action for which he would receive the Medal of Honor. The 21st Marines' advance had been halted by a network of Japanese pillboxes that rifle fire and grenades could not suppress. Williams, a diminutive West Virginian who had tried to enlist after Pearl Harbor and been initially rejected for being too short, took up a flamethrower and, protected by four rifle Marines who died one by one as the action progressed, spent four uninterrupted hours destroying pillbox after pillbox. He made repeated trips back to the rear to refuel his flamethrower, reenter the killing ground, and systematically eliminate each position. His action broke open the Japanese defensive line in the 21st Marines' sector and allowed the advance to continue.

The 3rd Marine Division's advance through the central plateau of Iwo Jima over the following weeks was a relentless, grinding ordeal. The 9th Marines, 21st Marines, and supporting elements pressed north against defenses that seemed inexhaustible. Every ridge contained more bunkers; every ravine concealed more Japanese soldiers who had to be killed one by one. Engineers from the 19th Marines worked constantly to keep supply lines open across terrain that defied conventional wheeled transport. The 12th Marines fired hundreds of thousands of artillery rounds in support of the infantry's advances, their forward observers working in conditions of extreme danger to direct fire onto targets often just meters from the Marine lines.

Private First Class Wilson D. Watson of the 9th Marines earned the Medal of Honor on February 26–27, 1945, by seizing a Japanese pillbox on top of a critical terrace, fighting off counterattacks through an entire day and night, and killing more than sixty Japanese soldiers in the process, almost single-handedly securing a position that allowed his battalion to advance. Watson survived the war and lived to tell his story.

Organized resistance on Iwo Jima ended on March 26, 1945, thirty-six days after the initial landing. In the costliest battle in Marine Corps history, the V Amphibious Corps suffered nearly 7,000 killed and more than 19,000 wounded. Of the Japanese garrison of approximately 21,000 men, fewer than 1,100 survived to be taken prisoner. The 3rd Marine Division's share of this carnage was severe — the division suffered thousands of casualties in its weeks of fighting, and many of its veteran units were badly depleted by the end of the campaign.

Fleet Admiral Chester Nimitz's tribute to the Marines at Iwo Jima became one of the most quoted observations of the Pacific War: that on Iwo Jima, uncommon valor was a common virtue. It was true of every division that fought there, but perhaps of no unit more than the "Fighting Third," whose men endured the hardest fighting in the island's fortified interior and whose sacrifice contributed decisively to the ultimate American victory.