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.

Germany, once dangerously dependent on Russian energy, has defied Russian expectations in its reaction to war in Ukraine.

A pending U.S. withdrawal from the Treaty on Open Skies has alarmed some allies in Europe, who fear losing a valuable means of monitoring Russia.

The collapse of the Cold War–era nuclear arms treaty signals trouble for U.S.-Russia relations, European security, and nonproliferation efforts.

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…