U.S. troops held air defense drills at a key air base outside Seoul as part of the large-scale Freedom Shield exercise, amid questions about how the possible redeployment of American missile defense assets could impact the military’s ability to counter North Korea.

The March 11 air defense drill involved the Patriot missile system and Avenger Air Defense System operated by the U.S. Army’s 52nd Air Defense Artillery Regiment alongside 7th Air Force troops, according to the Army.

The drill focused on readiness, crew proficiency and integrated air and missile defense against evolving threats on the Korean Peninsula, the Army said.