The Islamic State and Syria: American forces are engaged in major battles to liberate Mosul in Iraq and Raqqa in Syria from the Islamic State and are fighting extremists elsewhere, including Mali. Mr. Trump, who has said, “I know more about ISIS than the generals,” has not offered any plan beyond “I would bomb the [expletive] out of ’em.”

On Syria, he has talked of abandoning American support for rebels trying to oust President Bashar al-Assad and joining the alliance between Mr. Assad and Russia, Mr. Assad’s partner in bombing Syrian civilians. After 500,000 deaths, there’s no end to the five-year civil war, which has created chaos, allowing ISIS to thrive and claim large parts of Syrian territory. A united effort to fight ISIS would require a peace deal between Mr. Assad and the opposition forces. But Secretary of State John Kerry has not been able to get Russia to push Mr. Assad in that direction. Mr. Trump seems confident he can work with Mr. Putin, but it’s unclear that Russia would accept any deal unless Mr. Assad is allowed to remain in power indefinitely, which the Syrians he has brutalized are unlikely to accept.

Iran: Mr. Trump has vowed to tear up the 2015 deal under which Iran halted its most dangerous nuclear activities in exchange for the lifting of most international sanctions. The agreement is working, as many of its critics in Congress and the Middle East acknowledge. Mr. Trump, however, has chosen a national security adviser and a C.I.A. director who are both adamantly opposed to the deal, regardless of the consequences of ending it. If it is jettisoned, Iran would almost certainly resume its nuclear program. America’s partners in the agreement — Britain, France, Germany, Russia and China — will not reimpose sanctions as part of Mr. Trump’s quixotic quest for some “better deal,” and American businesses will be further disadvantaged in the competition for Iranian markets.

This is the kind of self-made crisis a new president cannot afford. Iranian moderates open to engagement with the West are battling for power against anti-Western hard-liners. The hard-liners hope to exploit Mr. Trump’s hostility to ensure that President Hassan Rouhani, who negotiated the nuclear deal, is defeated for re-election next year. It should matter to America which side prevails.

Nuclear Proliferation: Mr. Trump will soon command America’s nuclear arsenal. In the campaign, he spoke casually about possibly retaliating against the Islamic State with a nuclear weapon and about Japan, South Korea and Saudi Arabia possibly developing their own nuclear arsenals instead of relying on defense alliances with the United States. Since World War II, the United States has sought to prevent nuclear war and the expansion of nuclear arsenals. It would be catastrophic if a nuclear weapon is used during Mr. Trump’s presidency or if his stance encouraged more countries to acquire such arms.