Trident Juncture, NATO's largest military exercise in more than a decade, is underway as Russia's government deepens military involvement in Syria.

The military exercise is being hosted by Spain, Portugal, and Italy. Around 36,000 personnel, 60 ships and 140 aircraft from allied and partner nations are taking part in the event. Exercises are taking place at 16 locations on land, as well as in the Atlantic Ocean and the Mediterranean Sea. A video recently released by NATO shows Portuguese and U.S. Marines in Sebutal, Portugal, engaged in an amphibious assault.

"This sends a very clear message to any potential aggressor," said U.S. Air Force Gen. Philip Breedlove NATO's Supreme Allied Commander Europe.

NATO Deputy Secretary General Ambassador Alexander Vershbow noted it is the largest event of its kind in over a decade for NATO.

The exercise comes as NATO relations with Russia are at their most complex since the end of the Cold War. Russian President Vlaidmir Putin has pledged military support for Syrian President Bashar al-Assad, a regime fighting both the Free Syrian Army and the Islamic State, Sunni militants also identified as Daesh and by the acronyms ISIS and ISIL.

Russia recently signed a deal with the United States, a NATO partner, to avoid conflict during air operations in Syria.