Among the least remarked, but perhaps most disturbing, of Donald Trump’s disqualifications for leadership is his comprehensive ignorance of geopolitical history. In a president, ignorance is contagious — and potentially lethal. For Donald Trump seeks to implant an ethnonationalist “America First” sensibility which, by erasing 70 years of global reality, endangers us far more than any foreign enemy.

What Trump doesn’t grasp is basic World Affairs 101. The open world we live in arose from a transformative post-World War II vision — that America could lead in forging a more prosperous and peaceful international community, in which nations traded freely and eschewed aggression and oppression in favor of democracy and self-determination. This was often more dream than reality. The mediating institutions of this multi-faceted collaboration — notably the United Nations — were flawed and balky, and war, genocide, and humanitarian disasters have stained the decades since. But the concept worked well enough that cooperation, democracy, and prosperity increased around the globe.

Now come Russia and China. Russia murders and jails dissidents; China strangles dissent while choking the Internet. Russia has invaded Georgia, annexed Crimea, undermined the Ukraine, committed war crimes in Syria, launched cyberwarfare against electoral and other institutions in seven European countries, and attempted to influence America’s 2016 elections through systematic hacking. The Chinese assert dominion over the South China Sea in defiance of international maritime rights while failing to constrain the menacing nuclear program of its ally, North Korea. All this breeds the conflict and instability the framework of world order exists to prevent.

Our recent history is hardly blameless — America’s invasion of Iraq ultimately spawned the global threat of ISIS while destabilizing an already volatile region. But that is an argument for a rational world order, not against it. Indeed, our original sin in Iraq was invading a foreign country without sufficient justification or appreciation of the consequences.

Trump remains in the zero-sum environment of a venal developer.

Enter Donald Trump. At a time when the concept of international cooperation is essential, Trump proposes to blow it up. He scorns the idea that America has any interest beyond itself, no matter how pragmatic. He demeans the Paris climate deal, the Iran nuclear accord, NATO, the European Union, and free trade agreements. He says nothing about democracy and human rights — instead, he endorses torture and praises the murderous Vladimir Putin as “a strong leader.” Trump’s world order mirrors the world of his mind, a Hobbesian place subject to his narcissistic whims — and a dangerous place for us and everyone else.

Consider its potential outcomes: Russia dominating its neighbors against their will. Russia using Iran to destabilize the Middle East and spread Russian influence, exacerbating the Shia- Sunni conflict. Russia and the Assad regime slaughtering their way to “victory” in Syria, deepening the humanitarian disaster that also threatens the security of the Middle East, Europe, and ultimately the United States.

China would become the dominant economic, political, and military power in Asia, bending other countries to its will at whatever cost to them and us. The fragmentation of Europe would likely accelerate, another goal of Putin. America would retreat to a defensive crouch, our homeland security posture territorial, our trading relations fractious, our policies hostile to the immigration that has always renewed our energies. The instruments of international order would atrophy. And Trump’s engagement with the world, far from being thought out or systematic, would be spasmodic — random ad hoc responses to whatever threat emerges from our neglect.

And how would these responses be executed? Trump proposes drastically slashing our budget for diplomacy and foreign assistance in favor of beefing up the military — a trade-off deplored by generals who know that diplomatic failure means increased dangers which, quite likely, we would have to meet unilaterally. Somehow Trump imagines we can retreat behind walls, physical and economic, calling on a muscular military to squelch whatever crises may arise in the fatefully small world that eludes Trump’s crabbed imagination.

That real world’s gravest threats transcend borders: nuclear proliferation, transnational terrorism, WMDs, North Korean warheads, climate change, pandemics, cyberwarfare, global financial crises, economic dislocation, and mass humanitarian disasters. Only global cooperation can prevent the worst. Like a fly in amber, Trump remains in the zero-sum environment of a venal developer, where one prospers by threatening, blustering, and stiffing subcontractors. In this stunted mentality, our engagement with the world has cost us more than all we’ve gained — in prosperity, security, and the spread of human decency.

He may well succeed in proving just how wrong that is.

Richard North Patterson’s column appears regularly in the Boston Globe. His latest book is “Fever Swamp.’’ Follow him on Twitter @RicPatterson.