NEWPORT, Wales—Leaders of the North Atlantic Treaty Organization, reacting to Russia's intervention in Ukraine, agreed to establish a so-called spearhead force of several thousand troops designed to move into trouble spots at short notice.

The force of several thousand troops is aimed at increasing NATO's ability to move quickly into Central and Eastern Europe to deter Russia from intervening in former Soviet-bloc countries that are now members of the 28-nation alliance.

Leaders depicted it as one of a number of measures that would publicly reinforce NATO's commitment to Article 5 of its founding treaty, which states that an attack against one or more ally is considered as an attack against them all.

"Here in Wales, we've left absolutely no doubt—we will defend every ally," said U.S. President Barack Obama after the two-day summit closed.

The measures to bolster the alliance's eastern flank also include higher-tempo air patrols, continuing naval patrols, more frequent military exercises in the region, updates to the alliance's military plans and a pledge to reverse recent declines in military spending.