This week, President Trump is meeting with allied heads of state at a summit of the North Atlantic Treaty Organization (NATO). Among the many items on the agenda is the question of enlarging NATO to include other countries such as Ukraine. Although Russian aggression in Ukraine has been rightly condemned, those who urge for NATO to accept Ukraine as a full member are making a grave mistake.

If Ukraine joined NATO, it would become an even more unstable hotspot that America would be obligated to defend. Why should the U.S. risk war with a nuclear-armed Russia in Moscow’s backyard? NATO is a military alliance to defend Europe, not a democracy-promotion machine intended to reorder the political equilibrium in every European country. Though Washington may wish it, NATO cannot solve every problem nor can it smooth over all local flash points.

It’s easy to understand why some wish to bring Ukraine under the alliance’s security umbrella. After all, NATO has deterred Soviet and Russian aggression for nearly 70 years, and good Westerners who watched the Maidan protests have had their heart strings pulled. But expanding NATO means that if Ukraine asks for help in its current war, America’s sons and daughters will be called upon to die. If Trump and other administration officials asked American voters whether that’s something they want, the answer would be a firm “no.”

Furthermore, calls for Ukraine to join NATO forget that deterrence works because it relies on mutually assured destruction (MAD) and on some level of respect for each side’s national interests. When one side communicates that it no longer cares about the other’s security concerns, the likelihood of war skyrockets. For instance, in 1962, when Moscow put missiles in Cuba, America reacted very forcefully to get the Soviet Union to remove them—even though doing so brought the world to the brink. Furthermore, in 1983, when NATO staged its largest-ever exercises under Reagan—known as Able Archer 83—the Soviet Union thought it was a cover for an attack and nearly launched their own nuclear strikes as a result.

These same dynamics apply to Ukraine and the question of NATO accession. Although obviously the United States would never deliberately attack Russia, it doesn’t look that way from Moscow. Whether anyone likes it or not, Putin believes that Russia is reacting defensively and fears the possibility of a NATO-led overthrow of his government. He saw what happened when the Western-backed Maidan toppled Ukrainian President Viktor Yanukovych and thinks America might be tempted to do the same to him. As a result of this—and of Putin’s general revisionism—Russia is the spoiler for any Ukrainian conflict and would likely escalate the use of force to keep Ukraine out of NATO.

This is why U.S. deterrence wouldn’t apply as easily to Ukraine if it did start the process of joining NATO. If Russia was willing to annex Crimea and invade eastern Ukraine as a de facto veto on Ukraine’s NATO aspirations, it would certainly do far worse if official accession plans were announced. So far, NATO has pledged that Ukraine will one day join, but no such plans have been implemented. Additionally, it would likely take several years of reforms in accordance with a membership action plan before Ukraine could join NATO, which would give Russia time to react.

If Russia believes Ukraine is worth fighting for, then America and NATO need to deeply consider the implications rather than just push ahead for membership. To ignore this reality is to be naïve about how the world works. America cannot be the world’s crusader for democracy in every crisis. Where would that end? The argument that other countries’ interests do not matter and that the U.S. just needs to bring everyone under its protective umbrella collapses on itself. Reduced to its absurd logical conclusion, that would mean America should try to protect literally every state on the planet from aggression and dictatorship while also preparing to fight anyone—even nuclear powers—who gets in the way. The brutal truth is that the U.S. needs to protect its own democracy and prosperity. We cannot always save the day and Washington can no more deliver a perfectly happy ending to Ukraine than it could to Iraq.

As former U.S. ambassador to Ukraine Steven Pifer wrote, “Until the simmering conflict in the Donbas and frozen conflict in Crimea are resolved, Ukraine has little prospect of membership. Bringing Ukraine in with the ongoing disputes would mean that NATO would face an Article 5 contingency against Russia on day one of Kyiv’s membership.” Moreover, Henry Kissinger himself has urged that Ukraine ought to be considered a bridge between West and East rather than another potential NATO ally.

Washington needs to realize that NATO’s expansion is not always in America’s interests and that in this case the cost would be far too high. The United States should focus on holding NATO’s interest-based red lines while also recognizing Russia’s interests—challenging them where we must but not in every possible circumstance. The alternative would be for the Second Cold War to drag on longer than is necessary to the risk of all.

John Dale Grover is a fellow with Defense Priorities and a writer at Young Voices. He is also a graduate student at George Mason’s School of Conflict Analysis and Resolution. His articles have appeared in The National Interest, Forbes, Fox News, Real Clear Defense, and The American Conservative.