The deaths of U.S. service members after Iran attacked a base in Jordan followed a week of fighting that has steadily upped the ante and could trigger the resumption of all-out war.

The U.S. military has reinstated a naval blockade and bombed Iran for several consecutive days, concentrating attacks on coastal areas near the Strait of Hormuz. But airstrikes have recently extended to infrastructure, such as railways that could be used to ferry weapons.

At the same time, Iran has launched attacks on commercial ships and at its neighbors across the Persian Gulf region, targeting U.S. military assets. But Tehran has also hit energy infrastructure and even water desalination plants.

Still, fighting hasn’t been as extensive as it was during the initial phases of the war. But U.S. deaths previously represented a red line for President Donald Trump.

Early last month—before both sides signed a memorandum of understanding that has since collapsed—he confided to aides that he would consider ending the prior ceasefire and go back to war if Iran kills American troops, according to the Wall Street Journal.