The U.S. and South Korean militaries conducted their biggest field exercises in years last month and separately held joint naval and air force drills involving a U.S. aircraft carrier strike group and nuclear-capable U.S. bombers. North Korea claimed the drills simulated an all-out war against North Korea and communicated threats against it. The United States and South Korea have said their exercises are defensive in nature and expanding them was necessary to cope with the North’s evolving threats.