If U.S. troops find themselves swept up in a terror attack, the U.S. military has three words of advice: “Run, hide or fight.”

After the Brussels terror attacks earlier this week injured an American airman and his family, troops have been barred from traveling to the city without approval from their superiors.

“Specifically, unofficial travel to Brussels (leave, liberty, and special pass) is prohibited until further notice,” U.S. European Command said. “Those on official travel or emergency leave to Brussels will also require the approval of the first General/Flag Officer in the traveler or sponsor’s chain of command.”

To keep the 60,000 U.S. troops and civilian personnel stationed in Europe safe, the Department of Defense advises service members to first try to run and hide. Service members should fight if the situation demands it. Above all, service members must cooperate with local authorities and U.S. security responding to the attack.

Defense Secretary Ashton Carter told cadets at the U.S. Military Academy at West Point that the attacks were “a grim reminder of how serious are the dangers we face, how dangerous, the dangers that civilization and our country face, the challenges of this complex world.”

“No attack, no attack can shake our resolve to accelerate the defeat of ISIL,” Carter said. “I know that yesterday’s news only galvanizes our determination – yours, too — to serve our nation and join those who came before you in this mission of defending our people of the values were share.”