Sweden moved one step closer to joining NATO on Tuesday, after Turkey’s Parliament backed the move, underscoring how the war in Ukraine continued to reshape the military alliance nearly three years into the fighting.

President Vladimir V. Putin of Russia cited NATO’s eastward spread as one of his reasons for invading Ukraine. But instead of curbing it, Europe’s largest ground war since NATO was formed 74 years ago has revived and returned it to its Cold War roots as a war-fighting alliance.

Despite dwindling military supplies and competing crises, NATO members have vowed to sustain support for Ukraine. But military aid from the United States remains stalled in Congress and current and former European diplomats have expressed growing concern that a second Trump presidency could mean the abandonment of the government in Kyiv along with a gutting of the alliance.

Here’s a guide to NATO and how its role has shifted since Russia’s invasion of Ukraine:

What is NATO?

The mutual-defense alliance was established in 1949, after World War II, by the United States, Canada and 10 European countries.

The treaty for which the alliance is named has 14 articles by which all NATO members must abide. The most important is Article 5, which declares that an attack against one member state is an attack against them all.