On Friday, the referendum results were presented to the acting governor general of Papua New Guinea in Port Moresby, the seat of government that has controlled the province since 1975.
A few hours later, Papua New Guinea's prime minister, James Marape, arrived at a sports field on Bougainville Island, where he acknowledged the 98 percent vote in favor of independence — and hinted at difficult negotiations ahead.
Speaking in the native Tok Pisin language to several thousand ecstatic Bougainvilleans, many wearing traditional grass skirts, carrying spears and waving the islands' cobalt-blue flag, he emphasized the need for economic development.
Later, on social media, he said a peace agreement that led to the referendum must be honored, but he did not promise to support independence. "I must honor both flags, both people," he said on his personal Facebook account. "I have in my heart of hearts God will lead us to that place both Bougainville and PNG seeks."
The preliminary turnout for the nonbinding vote was 85 percent, a figure that could rise to 90 percent once checks are done, officials said, indicating an almost universal wish for secession.
"The result is so strong that PNG doesn't have much room to not start talking about independence," said Shane McLeod, a research fellow at the Lowy Institute, an Australian think tank.
Not since East Timor gained independence from Indonesia in 2002, following a United Nations-sponsored referendum three years earlier, has a part of the region taken such a big step toward nationhood. And as in East Timor, Bougainville's journey has been marked by bloodshed that was often ignored by the outside world.
Bougainville — which a French admiral named after himself in 1768 — was one of Papua New Guinea's richest provinces until a civil war broke out in the late 1980s over the spoils of wealth generated by the world's third-largest copper mine.
An estimated 10,000 to 15,000 people were killed. A naval blockade by Papua New Guinea eventually strangled the main independence force, the Bougainville Revolutionary Army, and the war ended in 1998 with Bougainville granted autonomy and a promise of a vote on independence.
Today, the mine is long closed and Bougainville is desperately poor. The power supply is unreliable, most residents rely on rainwater, and roads are regularly cut off during the frequent rainstorms. There is limited radio and Internet coverage across Bougainville and dozens of outlying islands that make up the province.
Like many Bougainvilleans, Collette Tsiperau, a geologist whose family fled the civil war and now lives in Port Moresby, wants to return home and help rebuild what she hopes will become the 194th U.N. member state.
"Today is a historic day in Bougainville," the 33-year-old said in an interview Friday. "It's a kind of liberation for us all."
The emergence of a new state further complicates geopolitics in a strategically significant region where China is testing the century-long dominance of the United States and Australia.
Through inducements such as loans and infrastructure spending, the Chinese government has been persuading Pacific states to sever ties with democratic Taiwan — which the Communist Party considers a renegade province — and switch diplomatic recognition to Beijing.
That effort has gained steam ahead of elections in Taiwan next month. The Solomon Islands, just southeast of Bougainville, abandoned Taipei in favor of Beijing in September, followed days later by Kiribati.
China's moves have worried officials in Australia and the United States, long the leading powers and significant aid donors in the South Pacific.
Japan occupied then-Australian-administered Bougainville in 1942. Fifteen months later, about 14,000 U.S. Marines and soldiers landed on Bougainville, where they found the jungle conditions even tougher than Guadalcanal in the nearby Solomon Islands, which they had invaded nine months earlier, according to Gen. Alexander A. Vandegrift, commander of the 1st Marine Amphibious Corps.
By the time Japanese forces surrendered in 1945, an estimated one-quarter of Bougainville's 52,000 civilians had died during the war.
Today, Bougainville has thriving fishing and cocoa industries and copper and gold deposits worth an estimated $58 billion, according to the Lowy Institute.
But a new state would need to develop laws and physical and commercial infrastructure to reopen the main mine and attract substantial foreign capital, experts said.
"They know they are not ready for full independence," Bertie Ahern, a former prime minister of Ireland who oversaw the referendum, said in a phone interview.
"What I would like to see is where they are in five or seven years. I don't see them being able to do it quicker than that."