PINHEIRO DA CRUZ, Portugal — U.S. Marines and other NATO troops staged a mock amphibious assault on Europe's coastline Tuesday as part of the alliance's biggest exercise in 13 years — an event intended to demonstrate its conventional military muscle and send a message of warning to potential foes.

The Trident Juncture training event being held through November in Portugal, Spain and Italy involves some 36,000 soldiers, sailors and airmen from some 30 countries, with more than 200 aircraft and over 60 ships and submarines.

It's a show of strength designed to send a message, organizers and analysts say, that NATO possesses the power to deter potential attackers, and it hasn't forgotten about southern Europe despite pressing concerns about Russia's activities on the alliance's eastern border.

Trident Juncture "sends a message (that) the alliance is strong (and) we're going to take care of each other," U.S. Rear Admiral Roy Kitchener, Chief of Staff of NATO Naval Striking and Support Forces, told The Associated Press at the amphibious maneuvers in Portugal on Tuesday.

The weeks of maneuvers are also testing the flagship NATO Response Force — quick-deployment units created in 2014 and due to be ready next year.

Huge U.S. Navy hovercraft came from the USS Arlington around 100 kilometers (60 miles) south of the Portuguese capital Lisbon, generating clouds of spray and noise as they delivered U.S. and Portuguese Marines and armored vehicles to the beach while U.S. Navy helicopters flew overhead.

In the imagined scenario, NATO was projecting force to help a friendly country that requested help to re-establish order.

The Trident Juncture exercise "is really about showing that NATO is still apt and capable to deploy a major military force to deter an attack," said Bruno Lete of the German Marshall Fund of the United States, a Brussels-based nonprofit research group. "The symbolism is very important."

He noted that NATO has devoted lots of energy recently to eastern Europe, and "now it is showing it is here to defend southern Europe too." NATO top brass are rethinking their strategy in southern Europe due to events in the Mediterranean, especially Libya and Syria, he said.

The NATO exercise Tuesday was not without mishaps. The beach was too steep for the hovercraft to land at their first attempt, requiring them to go back out to sea and try again, and the first two armored vehicles onto the beach got stuck in the sand.