The 500th day since Vladimir Putin’s invasion of Russia’s neighbor Ukraine passed last week with a potential end of the war seemingly nowhere in sight. Though the world expected the Ukrainians to quickly fold under the Russian war machine, they have done anything but. Indeed, they have proven themselves at worst to be the equals of the Russians, and with the aide of military equipment from the West, even capable of eventually ousting the invaders.

This surprising Ukrainian resolve is mirrored in their leader, President Volodymyr Zelensky. Just how much of the Ukrainians’ determined resistance can be credited to Zelensky may be impossible to calculate, but one thing’s for certain — the man has never wavered.

But the facts on the ground don’t simply change due solely to determination and courage. The longer this war drags on, the less likely Ukraine comes out of it whole. For all the help from western Europe and the U.S., the sheer difference in the size of the two combatants makes the prospect of a Ukrainian victory improbable. Despite his confident rhetoric, Zelensky knows this. He knows that without things changing, this slow war of attrition will drain Ukraine of her resources faster than Russia.

That explains Zelensky’s repeated request for his country to be admitted into NATO. It’s not as if NATO isn’t expanding; with Turkey’s recent acquiescence, NATO just gave Sweden the green light to enter the alliance. Finland joined a few months ago. From Zelensky’s perspective, neither Sweden nor Finland would be seeking NATO membership had it not been for Ukraine.

It’s hard not to sympathize with Zelensky, who is seeking to engage any outside alliances to help save his country. A clearly frustrated Zelensky recently blasted NATO members for their foot-dragging over any potential future membership offer. In a social media post, the Ukrainian president called it “unprecedented and absurd” to deny Ukraine’s membership. “Vague wording about ‘conditions’ is added even for inviting Ukraine,” he complained. “It seems there is no readiness to invite Ukraine to NATO or to make it a member of the alliance.”

However sympathetic some may be to Zelensky’s plea, NATO is right to reject it. The obvious logic is that NATO nations allowing Ukraine into the alliance would immediately and rightly be viewed as a declaration of war against Russia. The whole purpose of NATO is for all member nations to be obliged and ready to defend any other member from a foreign enemy. Many of the NATO nations have thus far effectively done all that they can via military aide and supplies.

Second, Ukraine’s entrance into NATO would actually serve to benefit Putin, as he’d use it as proof positive of his warnings that an expansion of NATO represents an existential threat to Russia. Talk about a massive rallying opportunity for Putin. Beyond that, some are deeply concerned that Putin would engage his nuclear arsenal if NATO entered the war.

Third, as Zelensky acknowledged to NATO: “We understand some [of our partners] are afraid of talking about our membership now. Nobody wants a world war.” Yes, the prospect of NATO entering the war would almost inevitably ignite a world war. Russia is not waging this war alone. How quickly would China take advantage of such a move to take Taiwan? Indeed, it would have the perfect excuse to do so.

While it is true that the U.S. and much of NATO are effectively engaged in funding an increasingly expensive proxy war against Russia, at least American soldiers aren’t over there dying. If peace is the desired end, then NATO membership for Ukraine is the wrong answer.