Whatever else happens in President Biden’s tenure, and no matter how long that tenure lasts, the events this week in Europe will ensure that his presidency is a consequential one. Russian aggression in Ukraine posed a historic challenge, both moral and geopolitical. Mr. Biden responded by pursuing the revitalization and growth of the North Atlantic Treaty Organization, the very institution whose purported expansionism had been the pretext for Russian President Vladimir Putin’s violation of Ukraine’s territorial integrity. The just-concluded NATO summit in Madrid produced both approval for the accession of Sweden and Finland to NATO and stepped-up U.S. troop deployments to Europe, including a first-ever presence in Poland. Instead of backing down or cracking up, as the Russian leader no doubt hoped and quite possibly expected, the alliance has stood up, with unprecedented military assistance to Ukraine. And now it has been further extended geographically and solidified politically.