Australia and Papua New Guinea’s prime ministers on Tuesday began trekking into the South Pacific island nation’s mountainous interior to commemorate a pivotal World War II campaign and to underscore their current security alliance, which faces challenges from China’s growing regional influence.

Australian Prime Minister Anthony Albanese received an elaborate traditional welcome when he arrived by helicopter at Kokoda Village with his Papua New Guinean counterpart James Marape.

The pair will walk 15 kilometres (nine miles) over two days along the rugged Kokoda Track where the Japanese army’s advance toward what is now the national capital, Port Moresby, was halted in 1942 in the wilds of the Owen Stanley Range.

“In forging a relationship of brothers and sisters, together as one we will go forward,” Albanese told Australian Broadcasting Corp as the pair set off from the village in tropical heat and humidity.

“We’re walking step by step, symbolising our two nations walking together,” Albanese added.

Marape said their “shared journey today should send the world a message” that Papua New Guinea wants peaceful coexistence.

Australia and its nearest neighbour, Papua New Guinea, forged closer defence ties in December when Albanese and Marape signed a wide-ranging agreement in the Australian capital, Canberra.

The signing was delayed by six months after a security pact between Papua New Guinea and the United States sparked riots in the South Pacific nation over concerns that Papuan sovereignty was being undermined.

Marape said in December that his government’s security agreements with the U.S and Australia did not mean he was siding with those allies in their strategic competition with China.

Albanese said Marape had assured him during at state dinner in Port Moresby on Monday night that Australia remained its “referred security partner.”

“This is a relationship that has never been closer, as symbolised by the fact that we’ll be walking side-by-side down the Kokoda Track,” Albanese told ABC in Port Moresby on Tuesday before flying to Kokoda Village.

Joe Biden had planned to become the first sitting U.S president to visit Papua New Guinea in May last year, in a sign of the region’s growing strategic importance in the power struggle between the U.S. and China. But Biden canceled the visit to deal with a debt crisis in the U.S. Congress.

On top of disappointing Marape last year, Biden offended the prime minister last week by implying that the president’s army aviator uncle, Second Lt. Ambrose J. Finnegan Jr., had been eaten by “cannibals” after his plane crashed in Papua New Guinea during World War II.

Marape released a statement on Sunday saying that his “people daily live with the fear” of unexploded bombs left behind by a war that they had been “needlessly dragged into.”

Marape and Albanese will trek to a war memorial in the town of Isurava, the site of a bloody battle where U.S and Australian troops fought the Japanese in August 1942.

Both leaders will commemorate Anzac Day at Isurava on Thursday, 25 April — the date in 1915 when the Australian and New Zealand Army Corps landed on Turkey in an ill-fated campaign that provided the soldiers’ first combat of World War I.