NATO's biggest military exercise in years just started, but Russia may be more worried about 2 countries that aren't members of the alliance

Trident Juncture officially started Thursday, with some 50,000 troops from all 29 NATO members and Sweden and Finland preparing for drills on land, sea, and in the air from the Baltic Sea to Iceland.

As a NATO Article 5 exercise, Trident Juncture "will simulate NATO's collective response to an armed attack against one ally," the organization's secretary general, Jens Stoltenberg, said this month. "And it will exercise our ability to reinforce our troops from Europe and across the Atlantic."

NATO has increased deployments and readiness in Europe since Russia's 2014 incursion in Ukraine, as countries there have grown wary of their larger neighbor.

Stoltenberg has said the exercise will be "fictitious but realistic." But Russia has still taken exception.

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