Believing that the Americans would ultimately attack from Davao Gulf and also anticipating that they would be eventually driven from the city, the Japanese also prepared defensive bunkers inland behind its perimeter where they could retire and regroup, with the intention of prolonging the campaign as long as possible.

Instead of the expected headlong frontal assault on the Japanese defenses, the plan called for securing a beachhead at Illana Bay in the undefended west, then driving eastward more than 100 miles (160 km) through jungles and mountains to strike from the rear.

The objective, which called for achieving surprise and pressing forward quickly and aggressively by the invading forces, Eichelberger deemed, could unhinge the Japanese both physically and psychologically.

The key to success involved the beachhead performance of the landing force and the ability of units to maintain the momentum of their attack, preempting Japanese reactions, and hopefully before the rainy season started, which would impede movement.

A sizable force—numbering about 8,900 men of Lt. Gen. Tokichi Hojo's 54th Japanese Independent Mixed Brigade (IMB)—had established strong defensive positions around Zamboanga City at the southern tip of the peninsula.

For two weeks, American infantry—ably supported by Marine aviation and naval gunfire—fought the Japanese along a 5 miles (8.0 km) front, in terrain so rugged that tanks could not be used, and in positions heavily fortified with deep earthen emplacements, barbed wire, minefields, and booby traps.

[1]: 596 On 23 March, after heavy fighting, the center of the Japanese line finally broke, and in the next three days, the 162nd Infantry continued eliminating resistance in the central sector.

[1]: 621  Starting on 5 April, Colonel Jerome's Marine aviators from Dipolog moved to the Malabang airstrip, and with targeting information from the guerrillas, proceeded to bomb the Japanese positions.

[1]: 621  Sibert, Woodruff, and Noble realized they had the opportunity to speed up the initial penetration of central Mindanao and quickly changed their plans to take advantage of the new developments.

On 22 April, the 31st Division waded ashore, with Marine Aircraft Group 24 arriving at Malabang to provide air support for Mindanao ground operations.

While it took just 15 days, despite severe heat and humidity and constant rain, with an entire division travelling 115 miles (185 km) and seizing the last major Philippine city under Japanese control, the real battle for Mindanao had begun.

[clarification needed] A chronicler for the 24th Division wrote: The soldiers of the 24th Infantry, considered the post-Davao operations to be the hardest, bitterest and, most exhausting battle of the ten island campaigns.

To the foot soldiers fighting in the Davao province, the word abaca was synonymous with hell...Countless acres around Davao are covered with these thick-stemmed plants, fifteen to twenty feet high; the plants grow as closely together as sugar cane, and their long, lush, green leaves are in a welter of green so dense that a strong man must fight with the whole weight of his body for each foot of progress...In the abaca fields, visibility was rarely more than ten feet.

While the infantry sought out the Japanese defenses, platoons and squads worked through the abaca and surrounding jungle to seek out enemy bunkers and spider holes.

At Libby Airdrome and the village of Mintal, some 5 miles (8.0 km) west of Davao City, the 21st Infantry Regiment was attacked on three sides by a numerically stronger enemy.

On 14 May, posthumous Medal of Honor awardee, Private First Class James Diamond of D Company fell mortally wounded as he was leading a patrol to evacuate more casualties when came under heavy attack.

He drew enemy fire while sprinting to an abandoned machinegun and was caught in a hail of bullets, but his sacrifice enabled his patrol to reach safety.

By 17 May, exhausted and bloodied, the 24th Division renewed its offensive, and this time, the 19th Infantry Regiment,[1]: 633–635  supported by Fertig's guerrillas, blew open the Japanese eastern flanks before capturing the villages of Tacunan, Ula, Matina Biao, Magtuod and Mandug on 29 May.

[1]: 635 Meanwhile, the 31st Division had forged ahead to the town of Kibawe on Highway 1, some 40 miles (64 km) away, since 27 April, with the 124th Infantry Regiment of Colonel Edward M.Cullen at point, where the first monsoon rains started creating havoc on the advance.

The town led to a supposed Japanese supply trail that twisted and turned south, until it reached the ocean shore village of Talomo in Davao City.

Firing from dugout positions, camouflaged spider holes with connecting tunnels, and virtually invisible pillboxes, the defending Japanese chose to die in place rather than retreat.

[1]: 646–647 While mopping up operations by small American units and Filipino guerrillas continued for some time, General Eichelberger announced the end of organized Japanese resistance.