Looking for perspective on this past year and the one ahead, I turned to several of the nation’s most experienced former military commanders. One of them put it bluntly: America is so divided politically at home that we are becoming vulnerable to our adversaries abroad.

America, these retired military leaders fear, is so divided right now that it might be difficult to mobilize the country for war, if that were necessary. The nation survives amid division and dysfunction now, when we’re at peace, more or less. But if America faced a serious threat abroad, say from a nuclear-armed North Korea, these domestic fissures could be paralyzing.

The shrinking space for governance worries me at year-end. The problem begins at the top: President Trump is the most unpopular president in modern times. He’s less admired than his predecessor, Barack Obama. He misreads the nation: The more divisive Trump has become – the more he picks at the nation’s scabs – the less the public likes him, according to polls. Yet Trump persists, playing to his base, with harmful consequences for the country.

Trump brags about how well the stock market is doing. Meanwhile, he attacks the FBI, the NFL and other groups he thinks it will be advantageous to impugn. The nation’s wounds get redder and rawer. But the polls suggest that the public overall isn’t buying it. Trump’s numbers remain low, and Republicans keep losing key races, as in Virginia and Alabama.

Trump took office with a narrow margin of public approval, but it’s been going the other way since last February, when disapproval of his performance rose above 50 percent. It has stayed there, hovering at roughly the current 56 percent disapproval rate since the summer, according to a composite of polls gathered by the website FiveThirtyEight. Just under 38 percent of the country approves of the president.

An ominous set of snapshots of America came in a study released by the Pew Research Center. Pew found that partisan divisions are now much more important than religious or educational ones in driving political views. The gap between Republicans and Democrats on key issues has increased from 15 percentage points in 1994 to 36 points now.

Sharp partisan divisions extend even to issues where factual evidence should be crucial. Pew found that only 27 percent of Republicans who said they had “high” scientific knowledge believed that climate change was causing either rising sea levels or harm to wildlife, compared with 75 percent and 73 percent of high-knowledge Democrats.

What worries me most is that, in Trump’s America, people seem increasingly doubtful that these divisions can be healed. A CBS News poll taken in June found that 55 percent of the country thought “people of different political views can come together.” By October, only 47 percent were optimistic, and 51 percent doubted that reconciliation was possible.

How does Trump’s divided America look to foreign eyes? A Pew study last spring found that global confidence in the U.S. president had fallen from 64 percent at the end of Obama’s tenure to 22 percent at the start of Trump’s. Those expressing “no confidence” surged from 23 percent to 74 percent.

Foreign nations have bet on America’s internal divisions before, but they’ve mostly been wrong. Abraham Lincoln’s persistence preserved the Union, even as some European nations thought America would splinter. Franklin Roosevelt helped keep America together during the 1930s when some left-wing and right-wing agitators were urging violence, and fascism and communism abroad seemed the wave of the future.

America’s vulnerability to manipulation was vividly described in a 1945 report by Britain’s Secret Intelligence Service, which reviewed that nation’s covert-action campaign starting in 1940 to push America into war. I reported on this history of “British Security Coordination,” or BSC, as it was known, nearly 30 years ago. The spymasters’ cynical assessment has haunted me ever since:

“In planning its campaign, it was necessary for BSC to remember … the simple truth that the United States, a sovereign entity of comparatively recent birth, is inhabited by people of many conflicting races, interests and creeds. These people, though fully conscious of their wealth and power in the aggregate, are still unsure of themselves individually, still basically on the defense and still striving, as yet unavailingly but very defiantly, after national unity.”

Trump is a defiant nationalist, and perhaps he hopes to be a unifier. But as this year ends, the numbers tell us that he has brought a level of division and disarray that should worry even his most passionate supporters.