Around 1,500 U.S. Marines and other navy personnel were involved, as well as 800 South Koreans.
North Korea condemned the exercises, calling them a rehearsal for invasion. Seoul and Washington officials said they have no intention of invading the North.
The drills will wrap up at the end of April.
For weeks, North Korea has threatened to attack the U.S. and South Korea for holding joint military drills and for supporting U.N. sanctions.
Washington and Seoul say they've seen no evidence that Pyongyang is actually preparing for a major conflict, though South Korean defense officials say the North appears prepared to test-fire a medium-range missile capable of reaching the American territory of Guam.
The Associated Press contributed to this report.