South Korea and the United States kicked off their annual joint military exercise on Monday to strengthen their combined readiness posture.

The Ulchi Freedom Shield exercise will be held from Monday through August 28, involving some 18-thousand troops from South Korea. 

The allies plan to carry out a simulation-based command post exercise as well as field training exercises, but this year’s event has been adjusted, with about 20 of the 40 field drills postponed to next month.

South Korea’s Joint Chiefs of Staff cited an ongoing heat wave for the adjustment, but it appears to have also taken into account North Korea’s opposition to the exercises.

South Korea and the U.S. have stressed that the main goal of the drills — to respond to North Korean threats — remains unchanged.

The joint exercise will be held alongside a four-day nationwide civil defense exercise that runs through Thursday.