Ask historians to name America’s greatest foreign policy blunders, and you’ll often hear a litany of misbegotten interventions – Vietnam, Iraq, Afghanistan and other wars that went awry. But some of America’s biggest failures have been errors of omission rather than commission – cases in which the strategic sin was not doing too much but too little, and in which being overly cautious eventually exacted a terrible price.

Keep this in mind as America’s debate on Ukraine aid reaches its climax: The Senate recently approved a new infusion of assistance, but it faces difficult odds in a fractured, Republican-led House. If Washington doesn’t provide the aid that keeps Ukrainian forces fighting, the fallout will be grave and global, and it will undermine U.S. policy for years to come.

If the U.S. stops supporting Ukraine, no one else can pick up the slack. Europe can provide (and has provided) generous financial assistance, but it can’t provide the warfighting equipment Ukraine desperately needs. The consequences of a cutoff would thus start to accumulate quickly. Ukrainian forces have been pressed out of Avdiivka, in the east, thanks to serious artillery shortages. Within months – or sooner – a shell-starved Ukraine would face agonizing decisions about how to deploy its dwindling military resources.

Even then, the country wouldn’t collapse quickly: Russia has consistently struggled to mount large-scale offensive operations. But Ukraine would start to lose slowly, having to surrender territory that it might never get back. Certainly, a Ukraine without U.S. aid would lack any viable path to winning this war or earning a peace that leaves the country militarily defensible and economically viable. The likely outcome, eventually, would be settlement imposed at the point of Russian guns.

Some analysts believe failure will magically strengthen the U.S., by letting it turn decisively to the Indo-Pacific theater. In reality, the defeat of Ukraine would create pernicious, wide-ranging effects.

That outcome would send a terrible message, to countries everywhere, about the resolve and strength of America relative to its autocratic adversaries. It would undercut momentum toward improving America’s defense industrial base, given that funding for Ukraine is mostly being used to expand production of weapons in the U.S.

A Ukrainian defeat would sow serious instability in Europe, as a vengeful, militarily mobilized Russia menaces NATO’s Eastern front. It would increase global pressures for nuclear proliferation, as vulnerable countries – Poland, South Korea and Japan, to name a few — conclude that the ultimate weapon is their only means of security and, perhaps, survival. Not least, a Russian victory in Ukraine would shatter the post-1945 norm against the forcible conquest and annexation of territory, thereby pulling the world back toward vicious anarchy.

Don’t buy the argument that cutting Ukraine loose would be a victory for sophisticated realism. It would be a triumph for the wing of the Republican Party that knows little and cares less about the indispensable role U.S. power has played in creating the relatively benign world America and so many other countries inhabit today. And given that the U.S. is currently preventing these dismal outcomes at a cost of just 5% to 6% of its annual defense budget – and without putting a single American soldier in combat – such a retreat would constitute one of the great unforced errors in the history of U.S. foreign policy.

If this sounds like an exaggeration, perhaps that’s because many critics of American policy are more interested in errors that come from overstretch rather than errors that come from under-stretch, even though the latter can more lethal than the former. It is easy to tally the undeniable costs, in blood and treasure, of wars gone haywire in Southeast Asia or the Middle East. Entire libraries are devoted to the trouble America finds due to excessive ambition. Yet the U.S. often gets itself, and the world, in even more trouble when it doesn’t lean forward enough.

In the 1920s and 1930s, the U.S. declined to use its power to stabilize the fragile, post-World War I system, and thereby helped bring on World War II. In the late 1990s, the U.S. declined to use force more aggressively against al-Qaeda – a mistake that contributed to 9/11 and all the travails that followed.

Or consider the history of U.S. policy toward Ukraine itself. After Russian President Vladimir Putin seized Crimea and invaded Eastern Ukraine in 2014, President Barack Obama held back lethal military assistance for fear of making that conflict worse. But in doing so, he helped convince Putin that he could dismember a neighboring country without paying much of a price.

It is understandable, perhaps, that a period dominated by unsatisfying U.S. military interventions in the Middle East produced an intellectual climate in which errors of commission are endlessly catalogued while errors of omission go comparatively unremarked. If ever there was a time to shift that mindset, it is now, as Washington approaches one of its most fateful foreign policy decisions in decades.