Russia’s moves to mobilize thousands more troops and to annex more of Ukraine’s territory signal a new, potentially more dangerous phase of the war.

The Madrid summit paved the way for Finland and Sweden to join and underscored that Russia is NATO’s chief security threat. But the alliance’s new defensive efforts aren’t likely to change Putin’s st…

President Putin’s deployment of troops in two separatist Ukrainian regions has nearly shut the door on diplomacy and intensified a showdown over European security.

Russia has demanded security guarantees that U.S. and NATO officials cannot accept, but there could still be room for building a sustainable dialogue on European security.

The first summit between Joe Biden and Vladimir Putin eased some of the tension in the fraught U.S.-Russia relationship. It also laid bare the difficult path ahead to resolving differences on many is…