Why would President Trump’s hardcore defenders think the best way to defend a floundering leader is to hurl repulsive dual-loyalty charges at a decorated Army combat veteran who feels an obligation to tell the truth to Congress?

Why would British Prime Minister Boris Johnson gamble on forcing an election in Britain at a time when his Conservative Party is under 40 percent in the polls?

And why are German Chancellor Angela Merkel’s center-right Christian Democrats and her coalition partners, the center-left Social Democrats, suffering electoral losses even though a large majority of the country wants her to serve out her term through 2021?

The standard answer to such questions focuses on political polarization, and there sure is a lot of it going around: left vs. right, urban vs. rural, religious vs. secular, young vs. old, prosperous vs. left-behind, pro-immigrant vs. anti-immigrant.

Polarization is deepened because many of these identities reinforce one another these days. To pick just one example underscored by recent studies from the Public Religion Research Institute and the Pew Research Center: Christian conservatives rally to the Republican Party while the secular are overwhelmingly Democrats. Partisans don’t just disagree about politics. They are divided by some of the most fundamental questions about human existence.

But another factor that we talk about far less is feeding the chaos: fragmentation. If some identities are mutually reinforcing, we have other commitments that split us into ever smaller groups. This feeds a tendency toward niche politics, visible in all the democratic nations. Taken together, polarization and niche politics make it very hard to forge the consensus required to solve problems and move democracies forward.

Consider first the sliming of Army Lt. Col. Alexander Vindman, who raised damaging questions about whether the White House summary of Trump’s phone call with Ukraine’s president omitted a reference to former vice president Joe Biden, whom Trump was pressuring the Ukrainian government to investigate.

Even before testifying Tuesday to House impeachment investigators, Vindman, the top Ukraine expert on the National Security Council, was subjected to attacks by Fox News and others on the far right because of his Ukrainian heritage. (He was brought to the United States by his refugee family when he was 3.)

Senate Democratic leader Charles E. Schumer (N.Y.) demanded Wednesday that Vindman be protected from retaliation. He “served our country for more than 20 years and is a recipient of the Purple Heart after being injured while serving in Iraq,” and yet “some have even gone so far as to call him a spy and question his loyalty to the United States.”

The vile assault on Vindman is designed to muddle a factual record highly damaging to the president. But it’s aimed at the Trump niche, roughly a quarter of Americans who will follow Trump’s lead on almost everything. This is a case of polarization and fragmentation reinforcing each other.

In Britain, the average of three recent polls gives Johnson’s party, pledged to leading Britain out of the European Union, just 38 percent of the vote. But Johnson has good reason to think he will win a majority of seats in the House of Commons, because his opponents are so fractured.

The Labour Party, broadly pro-E.U. but divided on the Brexit issue, is at just over 23 percent, while the passionately pro-E.U. Liberal Democrats stand at 18 percent. An additional 8 percent support pro-E.U. regional parties or the Greens, while the Brexit Party (committed to an even sharper break with Europe) polls at 11 percent.

This is a portrait of radical polarization (generally pro- and generally anti-E.U. parties are at roughly 49 percent each) and extreme fragmentation. Both make Britain harder to govern.

And in Germany, elections this past Sunday in the state of Thuringia saw both the Left Party and the far-right Alternative for Germany party gain ground — the left to 31 percent and the far right to 23 percent. The Christian Democrats and the Social Democrats each saw their vote reduced by a third, the Christian Democrats to 22 percent and the Social Democrats to a paltry 8 percent. The outcome in Thuringia is an extreme case of what’s going on in the country as a whole, but it is symptomatic of the broader decay of once unifying, middle-ground politics.

It would be nice to end on an upbeat note. But the economic and cultural forces pushing simultaneously toward polarization and fragmentation will be hard to overcome. And just when we need leadership that might promote solidarity and a degree of mutual understanding, the most powerful democratic country in the world is led by Trump, who thrives on chopping up our society into pieces.