Europe needs NATO. America needs NATO. You know who else needs NATO? Vladimir Putin.

The Russian leader has long used the existence of NATO to justify his antagonism toward the West.

Moscow's aggressiveness, you see, is merely a response to the "threat" NATO poses to Russian security. It's malarkey, of course – like a burglar claiming it's your fault he robbed your house because you had the audacity to buy a new TV.

Unlike Putin's Russia, NATO poses no threat of aggression. It is and always has been a purely defensive alliance. Even at the height of the Cold War, NATO harbored no designs on Soviet Russia and its satellites.

And once the Berlin Wall fell and the Soviet Union crumbled without a shot being fired, NATO welcomed new members to the alliance – contributing further to the mutual security of all and the expansion of freedom and democracy in Europe.

NATO and the new Russia lived peacefully side-by-side for years, until Putin embraced the fiction that, by increasing its membership, NATO was somehow encroaching on Russia and threatening its security.

Inside and outside the alliance, no one wants to pick a fight with Russia. Yet Putin's aggressiveness – from his invasions of Georgia and Crimea to his militarism in Ukraine – has made joining the alliance even more attractive.

And it's not just nations who've already taken casualties who seek membership.

In addition to Georgia and Ukraine, Finland and Macedonia are knocking on NATO's door, membership applications in hand. These countries and more rightly see NATO as a counter to Russia destabilizing adventurism.

No wonder Putin wants NATO to stop expanding. It crimps his style.

There is zero likelihood that Putin would stop harassing the alliance if NATO stopped taking in new members. Much like the czars of old, he wants a hard sphere of influence over Europe – something possible only if Moscow can break up NATO and decouple the U.S. from Europe.

And there is good reason for NATO to keep adding willing nations to its ranks.

In a defensive alliance, geography matters. A coherent frontier that keeps the bad guy further and further away is a good thing. Putin isn't going away anytime soon, so NATO must consider the geography of defending its eastern flank for the foreseeable future.

Freedom matters, too. Denying willing nations the right to participate in collective defense would only restrict freedom. Nor does it make any sense to discourage aspirant nations from meeting the political-military standards to qualify for NATO membership.

Know what doesn't matter? Size.

Many small states that have entered NATO have been net contributors to security. They have hit or are on track to hit the agreed upon NATO defense spending targets. They participate in NATO missions. In terms of manpower and operational contributions, they kick in more than they take out – easily outperforming many much bigger countries on a pound-for-pound basis.

Tiny Estonia is a case in point. It easily meets NATO's annual defense spending target of 2 percent of GDP.

Meanwhile, big, rich Germany struggles to do devote more than 1 percent of its GDP to defense; its finance minister recently proposed a budget that would actually reduce defense spending after 2020.

Today, Macedonia is poised to join the alliance. But the prospects of more nations joining the club anytime soon are dim.

Yet, NATO's open door policy is more than symbolism. It represents what NATO is: an alliance of free nation-states committed to mutual defense cooperation. As long as there are rulers like Putin, the need for that kind of commitment will not diminish.

Putin knows that. He fears that. It restrains him. This is no time for NATO to remove that restraint.