Organisers say about 1,800 representatives from 100 countries, including political, military and academic leaders, are gathering in Beijing for the Xiangshan Forum, considered China's answer to the annual Shangri-La meeting in Singapore.
The three-day event comes as China presents itself as a mediator of fraught global issues including ongoing wars in Ukraine and Gaza.
Addressing attendees at the opening ceremony on Thursday, Dong warned of "new threats and challenges" now facing world peace.
"While the themes of the times -- peace and development -- remain unchanged, the clouds of a Cold War mentality, hegemonism and protectionism have not lifted," he said.
"Historical memory must serve as a constant warning to recognise and oppose hegemonic logic and acts of bullying that are disguised in a new form."
The comments were a subtle reference to the United States, China's primary competitor in recent years across a wide range of economic and geopolitical arenas.
Dong's remarks come two weeks after a grand military parade in Tiananmen Square to commemorate China's 1945 victory over a Japanese invasion, which left millions dead.
The parade saw China unveil a host of new weapons, including advanced intercontinental ballistic missiles, drones and laser technology.
In attendance were several leaders that have long been at odds with Western governments, including Russia's Vladimir Putin and North Korea's Kim Jong Un.
- Maritime disputes -
Dong's remarks also touched on China's protection of maritime interests -- a topic that ruffles feathers in the region and beyond.
Several countries are currently ensnared in longstanding disputes with Beijing over sovereignty in the contested South China Sea.
Recent months have seen a series of confrontations between China and close US ally the Philippines in the crucial waterway.
Beijing claims almost all of the area despite an international ruling that the assertion has no basis.
"The so-called freedom of navigation advocated by certain countries outside the region and the so-called international arbitration advocated by certain claimants blatantly challenge the fundamental norms of international relations," Dong said.
He added that China's safeguarding of "territorial sovereignty and maritime rights and interests... is a firm defence of the post-war order and international rule of law".
The Philippines said this week that one person was injured when a water cannon attack by a China Coast Guard vessel shattered a window on the bridge of a fisheries bureau ship near a disputed shoal in the South China Sea.
The defence forum comes with anticipation growing over a potential meeting between Chinese President Xi Jinping and US President Donald Trump.
Dong held a video call this week with US counterpart Pete Hegseth, which covered various thorny topics including the South China Sea and Taiwan.
China considers self-ruled Taiwan part of its territory and has not excluded the use of force to take it.
Washington is the democratic island's main arms supplier and is committed to its defence.
The Pentagon said the talks were "candid and constructive", with Hegseth and Dong agreeing to further discussions.
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