If adversaries deem America to be as feeble as President Biden appeared during Thursday’s debate, will they use one of the lamest of lame-duck periods in history to undermine the country’s global leadership and attack our allies?

Whether Mr. Biden remains on the ticket in November or is replaced with another Democrat, his presidential term will last until January 20. After the CNN debate, Chairman Xi, President Putin, Supreme Leader Khamanei, and the North Korean dictator might conclude that a president whose faculties seem compromised represents an opportunity to weaken America.

One argument President Trump made Thursday is that world leaders do not respect Mr. Biden’s America. “He will drive us into World War III,” the former president said. A similar argument was made by Democrats about Mr. Trump before his 2016 victory.

In the last three-and-a-half years, though, a global coalition of dictators has tightened significantly. A powerful bloc of world leaders, whose only common goal is to diminish America’s power in the world, increased its cooperation. 

Iran and North Korea are helping arm Russia for its Ukraine war. Russia is boosting Tehran’s nuclear program. Communist China is buying subsidized oil from the Islamic Republic in defiance of American sanctions. Beijing is aiding Moscow financially and diplomatically. 

Moscow officials insist that Mr. Putin ignored the Biden-Trump showdown. “I don’t think that you can expect the Russian president to set his alarm to watch the debate,” the Kremlin spokesman, Dmitry Peskov, said. “This isn’t a main event for us.”

Still, the government-controlled Russian press was gleeful. “Biden expectedly misspoke multiple times and stammered, Democrats have already called his performance a failure,” a top press agency, RIA, writes. In Communist China, the Beijing-monitored social media outlet Weibo was just as snarky. 

Conspiracy theories were freely expressed regarding Mr. Biden’s condition, including unsubstantiated speculation about use of performance-enhancing drugs. “A normal person blinks every 3 seconds when he speaks,” according to one Weibo user, who asks, “Why does Biden take 20 seconds?”

Popular comments aside, Chinese, Russian, Arab, and Iranian observers noticed that Democrats and former supporters were shocked by Mr. Biden’s performance. Leaders must have heard a groundswell of calls on the president to withdraw from the race. 

Whether Mr. Biden remains on the top of the ticket or is replaced by another Democrat, he will remain president for nearly seven more months unless he resigns in favor of Vice President Harris. His debate image — one of a frail, at times incoherent, and slow-witted leader — could change calculations in world capitals. 

Mr. Xi is scheduled to go to Peru in the fall for an Asia-Pacific summit. The visit will follow the November 5 presidential election, and Mr. Biden is yet to confirm his attendance. During the event, the Beijing strongman will inaugurate a $3.5 billion Peruvian project, a megaport built by a Beijing-controlled firm, Cosco.

The deep-water port will tighten commercial trade between Communist China and Latin America, and seal the growing ties between Beijing and the resource-rich Western Hemisphere. It will also bring the People’s Liberation Navy ever closer to America’s backyard. In his own backyard, meanwhile,  Mr. Xi increasingly threatens the Philippines and other neighbors.

Beijing may well assess that a weakened American presidency until January is a ripe moment to fulfill Mr. Xi’s declared goal of annexing the island of Taiwan. Following Mr. Biden’s debate performance, he might conclude that America’s reaction would not be as forceful as the White House has declared it would be. 

In the last few months, similarly, the Islamic Republic has installed 1,400 new high-speed uranium enrichment centrifuges inside the underground Fordow plant, in defiance of global pressure. The International Atomic Energy Agency reports that Iran is amassing enriched uranium to 60 percent purity, which could easily be converted to weapons level.     

Will the aging Mr. Khamenei decide that Mr. Biden’s woes are a good time to complete the transformation of the Islamic Republic into a nuclear-armed power? Will President Kim of North Korea now seize the moment to attack South Korea? How about Mr. Putin’s designs on the Baltic states? 

Israelis are concerned that American weaknesses could harm their ability to fend off Tehran-backed armies that aim to grind them down by a war of attrition. Will Hamas in Gaza and the West Bank, the Iraqi and Syrian militias, the Houthis, and, most notably, Hezbollah now use America’s months of distraction to further escalate the war on the Jewish state?

Facing such perils, Mr. Biden could make a decisive military move in one or more of these arenas. If successful, it would also enhance his political fortunes at home. Yet, his performance Thursday raises questions as to whether he is capable of making such bold decisions.