For more than two years, Republicans have rightly excoriated President Biden for his calamitous withdrawal of US forces from Afghanistan. That shambles of a retreat not only was a disaster for the Afghan people, it was an announcement of American weakness that Vladimir Putin doubtless took as a green light to invade Ukraine six months later.

Republicans were equally scathing when Barack Obama did nothing after Syria’s dictator killed 1,400 civilians with chemical weapons — a crime Obama had warned would be a “red line.” That too broadcast a signal of irresolution that had disastrous results for America and the world.

Now it is Ukraine’s fate that hangs in the balance. Kyiv desperately needs a fresh infusion of ammunition and other aid if it is to succeed in beating back Russia’s brutal invasion. Ukrainians, who are fighting for their lives and liberty in the largest European war since 1945, have not asked any other nation to risk the life of a single soldier in Ukraine’s defense. What they have asked for is the weapons to do the job themselves — yet Republicans in Congress have refused to vote for another nickel of military aid unless the Biden administration and Senate Democrats first agree to new immigration restrictions at the US-Mexico border. Even worse, some hardline Republican populists, like Senators J.D. Vance of Ohio and Josh Hawley of Missouri have called for cutting off aid to Ukraine altogether.

During a press conference Tuesday, Biden said he was interested in compromising with Republicans on border policy in order to win their backing for $50 billion in new funding for Ukraine. Senate negotiators reported on Thursday that they were making progress toward a deal that they hoped could be finalized before Congress disperses for the holidays.

Otto von Bismarck, the first chancellor of Germany, called politics “the art of the possible.” In the current climate, perhaps it is politically possible to sustain Ukraine’s fight against Russian aggression only by making it harder for migrants to find refuge in the United States. But on the merits, linking the two is irrational.

America’s border and immigration laws desperately need reform. But the changes Republicans are demanding — empowering border agents to rapidly expel undocumented migrants, sharply increasing the barriers to asylum, and forcing people seeking humanitarian parole to wait in Mexico until their cases are heard — are not reforms. They will only reinforce the fundamental problem with existing immigration policy: Demand for labor in the United States is extremely strong, yet for most would-be immigrants seeking work it is impossible to enter the country lawfully. “Trying to reduce illegal migration by making legal entry harder,” George Mason University law professor Ilya Somin wrote a few days ago, “is much like trying to fight Prohibition-era bootlegging by making it harder to obtain alcohol legally.”

For Republicans and the White House alike, immigration politics have become overpowering, as two new surveys attest. A Wall Street Journal poll finds that 64 percent of voters disapprove of Biden’s handling of border security, while in the latest CBS News poll, 1 in 5 voters rank “immigration and the border” as the most important problem facing the country. Only inflation was deemed a more pressing concern.

For all that, it is a mistake to entangle military funds for Ukraine with the turmoil at the Mexico border, two crises with no logical or strategic connection. The urgency of standing with Ukraine in its life-and-death struggle against Vladimir Putin’s Russia has not diminished. Ukraine is not just an ally of the democratic West confronting a revanchist, anti-American dictatorship. It has proved to be a formidable combat force that has used funds and arms supplied by America and Europe to decimate Moscow’s military threat.

Citing a declassified American intelligence assessment, The New York Times reported Tuesday that Ukraine has inflicted “staggeringly high” losses on Putin’s forces. For example, the Russian army had 360,000 troops at the start of the war. But it has suffered so many casualties — 315,000, or 88 percent — that Putin is now forced to recruit troops from Russian prisons.

“Moscow’s equipment has also been crushed,” noted the Times. “At the start of the war, Russia had 3,500 tanks but has lost 2,200, forcing them to pull 50-year-old T-62 tanks from storage.”

The return on the military aid invested in Ukraine has been spectacular. The most menacing threat to European security and stability — Russian aggression under a dictator bent on empire-building — has been dramatically reduced. Thanks to the combination of Ukrainian bravery and Western munitions, Poland, Lithuania, and other frontline NATO allies are safer today than they were two years ago, when Russia’s murderous assault began. And all without the loss of a single American soldier’s life.

It is true that Ukraine’s much-anticipated counteroffensive last summer didn’t succeed. But there are setbacks in all wars. For Ukraine and its Western supporters, victory against Russia must remain the goal. Putin cannot be allowed to conclude that if only he holds out long enough, Washington will eventually abandon Ukraine as it eventually abandoned Afghanistan and Vietnam.

The $46 billion in US military aid sent to Ukraine so far is a lot of money. But it amounts to hardly more than a rounding error next to the $12 trillion the federal government has spent since the war in Ukraine began. The costs have been far outweighed by the gains: American deterrence, after taking a body blow in Afghanistan, has been boosted. China’s belligerence toward Taiwan has been kept in check. NATO has been doubly strengthened: Member states have hiked their defense budgets, and Finland and Sweden applied to join the alliance.

According to Kremlin loyalist Valery Fedorov, it is likely that no more than 15 percent of the Russian public strongly supports Putin’s war of conquest against Ukraine. By contrast, a solid majority of Americans remains in favor of aiding Ukraine. In the most recent nationwide survey by the Reagan Institute, 67 percent of respondents expressed support for backing Ukraine and 76 percent said it was important to the United States that Ukraine prevail.

When Iraqi dictator Saddam Hussein invaded and annexed Kuwait in 1990, Margaret Thatcher, the British prime minister, gave then-president George H.W. Bush some memorable advice: “Remember, George, this is no time to go wobbly.” Bush took that advice to heart, and assembled an international coalition that liberated Kuwait.

It’s still good advice. Ukrainians have fought tenaciously to preserve their democracy and independence in the face of a devastating Russian onslaught. Providing the funds they need to keep fighting is manifestly in America’s national interest. With or without an immigration deal, Washington must not leave Ukraine in the lurch. This is no time to go wobbly.