U.S. Marines and other NATO troops have staged a mock amphibious assault on Europe's coastline as part of the alliance's biggest exercise in 13 years.

The Trident Juncture training event being held through November in Portugal, Spain and Italy involves about 36,000 soldiers, sailors and airmen from more than 30 countries.

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

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

Troops stormed a section of the southern Portuguese shoreline Tuesday, using hovercraft and helicopters.