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It felt like a scene from the Cold War, a perilous episode from a bygone era. An unpredictable Russian leader was amassing troops and tanks on a neighbor’s border. There was fear of a bloody East-West conflagration.

Then the Cold War turned hot: Vladimir V. Putin’s ordered Russian forces to invade Ukraine. The repercussions were immediate, and far-reaching.

Now, following the launch of Russia’s full-scale invasion on Feb. 24, the largest mobilization of forces Europe has seen since 1945 is underway. So far, Moscow has been denied the swift victory it anticipated, and has failed to capture major cities across the country, including Kyiv, the capital. It has been weighed down by an ill-prepared military and has faced tenacious resistance from Ukrainian soldiers and civilian resistance fighters. Still, Russia has superior military might, and Mr. Putin has indicated that his ultimate goal is to capture Kyiv, topple Ukraine’s democratically elected government, and subsume the country into Russia’s orbit.

The invasion threatens to destabilize the already volatile post-Soviet region, with serious consequences for the security structure that has governed Europe since the 1990s. Mr. Putin has long lamented the loss of Ukraine and other republics when the Soviet Union broke apart. Now, diminishing NATO, the military alliance that helped keep the Soviets in check, appears to be part of his mission. Before invading, Russia made a list of far-reaching demands to reshape that structure — positions NATO and the United States rejected.

With the war grinding on, U.S. intelligence agencies say Mr. Putin has been frustrated by the slow pace of the military advance and Russian commanders have been increasingly intensifying indiscriminate attacks on civilian targets and infrastructure and resorting to tactics used in previous wars in Chechnya and Syria. Mariupol. Kharkiv. Chernihiv. Sumy. Okhtyrka. Hostomel. Irpin. The list of Ukrainian cities turned to ruins keeps growing.

The war has unleashed a devastating humanitarian toll and claimed thousands of lives. It has also prompted more than three million people to flee Ukraine, spurring what the United Nations has called the fastest-growing refugee crisis since World War II.