Ukraine is staring down one of the world’s largest militaries deeply entrenched deep within its territory, but Kyiv may face another enemy that’s perhaps equally dangerous in the long run: the weakening of Western resolve. Top Russian officials predicted Monday that the U.S. and NATO are in the early throes of Ukraine “fatigue.” They said domestic politics on both sides of the Atlantic may be opening the door to a slow erosion of the steady economic and military aid that has kept Kyiv alive in the fight. Such accusations from the Kremlin are hardly new. Russian President Vladimir Putin banked his Ukraine gambit on the presumption that Western allies would eventually crack and abandon Ukraine. That prediction hasn’t panned out so far in the 19-month-old conflict. European Union foreign ministers on Monday traveled to Kyiv and met with Ukrainian President Volodymyr Zelenskyy. Both sides offered reassurances that the anti-Russian alliance has not frayed.

The White House was also eager to shoot down the idea that allied unity was faltering or that Russia could outwait the coalition assembled against it. “There is [a] strong, very strong international coalition behind Ukraine,” White House press secretary Karine Jean-Pierre told a briefing. “And if Putin thinks he can outlast us, he’s wrong. He’s wrong.”

But the unity of the U.S. and its leading allies is showing early signs that support for Ukraine won’t last indefinitely. In the U.S., leading candidates for the Republican presidential nomination, including former President Donald Trump, have taken a skeptical stance on open-ended American aid for Ukraine. Congressional Republicans over the weekend pulled Ukraine aid from a last-minute spending bill that averted a government shutdown.

The turmoil is increasingly evident in Europe as well. In Slovakia’s elections over the weekend, former Prime Minister Robert Fico and his Smer party emerged as the top vote-getters. Mr. Fico has said he wants to end the country’s military aid to Ukraine and favors immediate peace talks between Kyiv and Moscow. That stance echoes the positions of some Republican presidential candidates who say it’s time for Washington to use its leverage to push for peace rather than open-ended war with a ballooning price tag. Some analysts caution not to read too much into those developments and predict Western support for Ukraine will continue. They do acknowledge a growing reason for concern, especially given the virtually nonexistent appetite in the U.S. to involve itself in another yearslong war thousands of miles away. “I think the fear is well-founded,” said Jim Townsend, former deputy assistant secretary of defense for European and NATO policy during the Obama administration. “Whether it’s Afghanistan fatigue, Iraq fatigue, Balkans fatigue. Fatigue is a naturally occurring condition in any kind of warfare, particularly involving the U.S. or other democracies. We get fatigued pretty fast. We want quick results.”

The U.S. has given about $44 billion in direct military aid to Ukraine since the conflict began, in addition to other economic and humanitarian assistance. The Biden administration is pushing for more, and additional money for Ukraine had been a central part of the spending legislation fight that divided Republicans on Capitol Hill. The EU and its member states have given Ukraine $88 billion in total aid since the start of the war in February 2022, the bloc said in a fact sheet released late last month. At least $27 billion of that went directly to the Ukrainian military. That money has paid dividends. Western aid, equipment and military training helped Ukraine fend off Russia’s push toward Kyiv in the early days of the war and helped the Ukrainians notch several significant successes since. Western vehicles, artillery, counterdrone weapons and other tools have been vital to modest gains in Ukraine’s counteroffensive. Moscow is playing the long game, betting the U.S. and Europe will grow tired of war. Russia plans to dramatically increase its military budget next year, and officials in Moscow are anticipating that fighting in Ukraine will continue at least until 2025, according to documents that were apparently leaked from the Kremlin’s Finance Ministry. Russia’s defense spending is set to surge to about 30% of total public expenditures in 2024, British defense officials said Sunday, sketching out Russia’s vision for a war that drags on for years.

Kremlin spokesman Dmitry Peskov suggested that Western resolve is starting to flag. “Fatigue over this conflict — fatigue from the completely absurd sponsorship of the Kyiv regime — will grow in various countries, including the U.S.,” he said Monday. Europe shows troubling signs beyond the election results in Slovakia. Hungarian Prime Minister Viktor Orban has argued against providing Ukraine with arms. Just last week, he cast doubt on the notion that Ukraine could join the EU. In Poland, President Andrzej Duda has had to walk back comments from his prime minister that Warsaw was cutting off weapons trade with Ukraine in reaction to a flood of Ukrainian grain imports undercutting politically influential agricultural interests in Poland. As a neighboring country and a key thoroughfare for military aid to Ukraine, Poland is a crucial cog in the Western alliance. With Russia preparing for years of war and showing a willingness throughout history to send waves of bodies into a fight, Western solidarity will be more important than ever. “You outlast them,” said Mr. Townsend, referring to Russia. “Putin is certainly expecting the West to be as feckless as the West can be when they’re in the fight and they lose stomach for that fight, whether it’s Vietnam, Afghanistan or Iraq.”