 On Sunday, North Korea warned of what it called overwhelming countermeasures against trilateral drills, including the U.S., South Korea and Japan, which wrapped up on Saturday. North Korea's foreign ministry said in a statement Sunday that aggressive moves like the drills are forcing North Korea to develop more advanced weapons and increase military cooperation with other nations. That's an apparent reference to North Korea's signing of a treaty with Russia last month, obliging the two countries to give military assistance if either is attacked.