U.S. Soldiers with the 2nd Cavalry Regiment and Polish soldiers from the 6th Airborne Brigade pull security before navigating to a observation point July 17, 2015, near Nowa Deba, Poland. The exercise was part of Operation Atlantic Resolve, focused on joint training and security cooperation. (Brandon Anderson/U.S. Army)

U.S. Army Reserve combat engineers from the 374th Engineer Company tread water during Combat Water Survival Training at Fort Hunter Liggett, Calif., July 17, 2015. The training was part of a two-week field exercise known as Sapper Leader Course Prerequisite Training. (Michel Sauret/U.S. Army)

A U.S. Army Reserve combat engineer from the 374th Engineer Company covers a sector of fire during a simulated air assault landing, July 18, 2015, at Camp San Luis Obispo Military Installation, Calif. (Michel Sauret/U.S. Army)

Soldiers with 503rd Infantry Regiment, 173rd Airborne Brigade Combat Team, stack up near a simulated door at the Central Training Area, near Tapa, Estonia, during Operation Atlantic Resolve, July 19, 2015. Throughout the day, soldiers practiced building and setting the various charges they had learned about the day prior. (Juana M. Nesbitt/U.S. Army)

A U.S. Army Reserve combat engineer from the 374th Engineer Company checks his rope before rappelling a 100-foot tower during Sapper Leader, a two-week field exercise at Camp San Luis Obispo Military Installation, Calif., July 19,2015. (Michel Sauret/U.S. Army)

Two New York Army National Guard Soldiers from the 427th Brigade Support Battalion prepare to attach a sling load hook to a CH-47 Chinook helicopter, July 19, 2015, at Fort Drum, N.Y. (Alexander Rector/U.S. Army)

U.S., African and European servicemembers stand in formation during the opening ceremony for Western Accord 2015, a command post exercise, at Harskamp, The Netherlands, July 20, 2015. (Marcus Fichtl/U.S. Army)

Ukrainian and U.S. officers review troops in formation during a ceremony marking the start of Rapid Trident 2015, a long-standing cooperative training exercise focused on stability and peacekeeping operations, in Yavoriv, Ukraine, July 20, 2015. (Alex Skripinchuk/U.S. Army)

Maryland Army National Guard Soldiers freefall from 13,000 feet during a jump with the 158th Cavalry Regiment, on July 18, 2015, over Little Orleans, Md. (Margaret Taylor/U.S. Army)

Soldiers with 2nd Battalion, 503rd Infantry Regiment, 173rd Airborne Brigade, join their Estonian counterparts while practicing landing procedures in preparation for a joint airborne jump at Tapa Army Base, Estonia, July 21, 2015, as part of operation Atlantic Resolve. (Jacqueline Dowland/U.S. Army)