Oslo, Norway—In a few weeks’ time, the beautiful Norwegian fjords and mountains will witness NATO’s largest military exercise since the end of the Cold War.

The exercise, “Trident Juncture 2018,” will take place in a background of skepticism over America’s intentions with the transatlantic military alliance and heightened tensions with Russia.

Trident Juncture will involve more than 45,000 soldiers from all 29 NATO countries plus troops from Sweden and Finland, which aren’t part of the organization but have historically been close partners. More than 150 aircraft will compose the aerial component of the exercise. Sixty ships will furnish the naval element. More than 10,000 tanks, armored vehicles, and artillery batteries will provide the land portion.

The exercise will begin at the end of October and last until the first week of November. Participating aircraft will now be limited to Norwegian airspace, but will also venture into Swedish and Finnish skies.

Trident Juncture’s scenario will be “fictitious, but realistic [and] simulate NATO’s collective response to an armed attack against one ally.”

Under Article 5 of NATO’s constitution, all member states are obliged to render support in case one of their numbers is attacked. The only time Article 5 was invoked was after 9/11, when the United States was attacked in an unprecedented terrorist attack.

“Much of the exercise will happen in someone’s backyard, we will conduct the exercise amidst the Norwegian population. It is not something usual, but very realistic. We made sure to also involve environmental specialists,” said a Norwegian military officer about the exercise.

In a recent tweet, U.S. Secretary of Defense James Mattis said, “NATO Allies have renewed their sense of urgency on all aspects of burdensharing [sic]: Cash, Capabilities and Contributions. New U.S. funding reflects its commitment to European security.”

Although no NATO official has claimed it, Trident Juncture is a response to the recent Russian Vostok (East) military maneuver. According to the Russian defense ministry, Vostok involved more than 300,000 troops, 36,000 armored vehicles and tanks, 1,000 aircraft, and 80 ships. In a quite surprising move, China and Mongolia also participated in the exercise.

The Russian government raised its objections with the exercise through its deputy foreign minister, Maria Zaharova: “The escalation of NATO’s military and civilian activity in the Arctic region, that is, Russia’s immediate environment in the territory of Northern Norway, has not gone unnoticed. Developments in this region can have serious consequences and not our own fault.”

NATO Secretary-General Jens Stoltenberg responded by highlighting the exercise’s transparency, “all members of the Organization for Security and Cooperation in Europe, including Russia, have been invited to send observers.”

A Norwegian military spokesperson added, “despite the current political climate, we are concerned not to create any situation up North that could lead to any kind of misunderstanding.”

One of the exercise’s goals will be to test and approve NATO’s Spearhead Force, a new quick reaction force with a 48-hour deployment notice.

Norway, a founding NATO member, shares a land and water border with Russia.