The indispensable role of the US and NATO.
The first Secretary-General of NATO, General Sir Hastings Ismay, stated that the purpose of NATO is:
>"To keep the Americans in, the Russians out, and the Germans down."
He would go on to later say that:
>"I am convinced that the present solution is only a partial one, aimed at guarding the heart. It must grow until the whole free world gets under one umbrella."
The genius of the conception of NATO was that it served multiple purposes all at the same time. Confronting the Soviet Union was as much a concern as was the possibility of confronting Germany again in the future. The initial construct, in 1947, before NATO itself was an alliance between Britain and France against Soviet of German future threats, under the Treaty of Dunkirk, which was then expanded to include the Benelux nations under the Treaty of Brussels. In 1949, NATO itself came into existence with the addition of the USA, Canada, and some other European nations, notably including Italy as an effort to mitigate the communist party influence within that nation. Nevertheless, NATO didn't really take form as a practical matter until the Korean War, in 1950. From there is where dates the unified response of the West to the unified spread of communism then occurring. West Germany was not admitted until 1955.
The addition of West Germany was what created the Warsaw Pact, in the Soviet response after itself being rejected for membership in 1954.
None of these treaties came about easily, with France withdrawing, if in name only, over the admission of and consequences of of US membership. The German admission can be credited to a number of factors, chief among which were the need for its troops and its central location to any conflict with what was by then the Eastern Bloc. It was aided though, in mollifying misgivings concerning a martial Germany, by the previous admission of Italy, which had been successful in limiting communist aspirations within that nation. NATO needed Germany, but it could also look upon it as keeping nations' enemies close in a way the alliance was justifiably unwilling to do with the Soviet Union.
That said, only time has caused the German threat to be perceived less seriously, though the bankrupting of Greece and Portugal, largely via German economic policy, have provoked concerns reminiscent of those expressed within prior eras over German militarism. Coming upon the reunification of Germany, and the formation of the EU, those concerns have not proven unfounded. If, and when, the EU dissolves, as I expect it will, it will largely be over Germany, and France to a lesser extent, conducting themselves economically as had they conquered the continent together some time previously. The EU is a successful economic alignment, while a political disaster. If proof were needed, its response to COVID-19 has shown that the EU cannot be a successful political alignment, despite assertions to the contrary.
Aside from, if grudgingly on occasion, having successfully confronted the Soviet Union, NATO has largely kept the peace within Europe itself. Absent NATO, and the US membership in it, the Balkan civil wars would yet rage today, assuming there anyone yet left alive within the region. Left to its own devices Europe was wholly feckless in confronting civil war, even within its own backyard.
The strength of the US within NATO, aside from its obvious wealth and military assets, is the ability to prevail upon the continent towards doing the right thing, lest the US be forced to return to Europe under less than desirable conditions. NATO has been successful because it gives every European nation, and every European, reason to remember that they'd all be speaking German had the US not settled Germany's hash within the last century, twice.
I, like many people, thought there no further need for NATO once the USSR fell. As it turned out, the need for NATO not only continued, but escalated with the demonstration of European fecklessness in settling something as simple as was the breakup of the former Yugoslavia, which did involve the return of US forces to Europe. The world became a more, not less, dangerous place when the second superpower was no more. As it turns out within Europe, no, they cannot all just get along. It is NATO, and almost NATO alone, which keeps a lid on that.
It's the expansion of NATO, and nowhere more than in Poland, which not only keeps European peace, but keeps the threat from a resurgent Russia at bay. The Germans waffle on everything not directly in their own economic interest, while the US and NATO see what had been troops and capabilities stashed in Germany deployed to Poland. The Russians have offered ample evidence for what that should be so.
NATO would be worth preserving just because the Russians want to see it fractured. Russia's actions of recent have driven non-NATO countries Finland and Sweden into conducting joint naval exercises with the US and other NATO countries. That's a tacit endorsement for the future of NATO. And we owe that to Nordstream 2, which is the kind of German-Russian idea which reminds every resident of Europe of nothing so much as the Molotov–Ribbentrop Pact of 1939.
What the US gets out of it is not only peace in Europe, which is a price we twice paid last century, but peace beyond Europe. Unlike many, I've no objection to adding Ukraine and Georgia to NATO and the EU, though the latter being purely optional. Finland and Sweden would join today were it a presented option. The Russians would object, they see Europe from a more historical and divided perspective than do I, but then the Russians haven't made a cultural or economic advancement within a few centuries but that it was provoked in response to the West.
What it isn't is a question of whether the US would come to the defense of the Ukraine under an application of Article 5 (which has only been declared but for an attack upon the US) but whether Russia would actually provoke the question as a practical matter. We understand it still sorely chuffed NATO absorbed the Warsaw Pact whole, but observe that thing have never been better as a result. It isn't a question of whether the US and NATO would send troops to fight within the Ukraine, though they ultimately would if necessary, but whether NATO would be the rubric under which Western Europe embargoed Russia directly into poverty. Russia can bluster all it wants, and play the strategic board like a game of chess, but what it cannot do is tolerate the stopping of the flow of oil and gas from it into the West.
NATO is the only organization which can back up such sanctions. NATO is the only organization which can counter Putin's brinksmanship with the knowledge that Putin's position within Russia is by no means permanent. Europe can get its energy elsewhere and NATO, unlike the EU, is the political organization which can ensure it does so. And it can because the US is a member. The US would face no difficulty at all in convincing certain Middle Eastern nations rich in oil and gas that it would be in their interest to help it pump the Russian autocracy into submission over its grand desire to dominate the Ukraine. Saudi Arabia quietly helped to end the Cold War in just that manner, making Soviet debt unsustainable. The US wouldn't even have to remind it that hurting Russia hurts Iran.
The irony of NATO is that everyone resents, if not hates, the Americans, but the Americans serve an indispensable purpose in European affairs, namely that they not all kill each other. NATO is the vehicle by which that is done, and it has here to date not required much in the way of shots being fired.
While we're on the subject, we're also upon the brink of seeing a discussion concerning the revival of, and expansion of, a now dissolved SEATO. Viet Nam would join in a heartbeat, as would Taiwan, Singapore, and Malaysia. No nation need enjoy being beholden to the US, or having a need to be deferential to the US, in order to see what an alliance with the US via an organization like NATO has done for its members. As for the US, it's gotten peace at a bargain basement price for its efforts.