European Union leaders meet in Brussels on Thursday, hoping to approve a landmark multibillion euro fund for Ukraine that will help keep the country afloat for the next four years, no matter what happens on the battlefield, or in the U.S. Congress threatening to cut support.

The only thing standing in their way is Prime Minister Viktor Orban of Hungary. Again.

A compromise with Mr. Orban, who has demanded an annual veto on the spending, has remained elusive, meaning that the unanimity required for such a deal among the 27 E.U. states still seems out of reach. If Mr. Orban continues to stand in the way, E.U. leaders have made clear they are ready do whatever is necessary to support Ukraine and are prepared to work around him — or even to punish him.

Yet even if the remaining 26 leaders are not forced to go ahead without Mr. Orban, a larger problem is now firmly front and center: What will the E.U. do about its Hungary problem?

For a small country that accounts for just 1 percent of the bloc’s economic output, Hungary has been a big headache.

It has been at loggerheads with the E.U. for years over its transgressions against E.U. norms and values pertaining to the rule of law. And it has consistently slowed, shaved or stymied a range of European ambitions, including some sanctions against Russia as well as Sweden’s bid to enter NATO.