South Korea has halted all trade with North Korea and announced it will seek U.N. sanctions (which N. Korea has previously threatened to interpret as an act of war). The White House responded decisively, promising "unequivocal" support to S. Korea. The U.S. has vowed to back all measures requested of the U.N., bolster S. Korean defenses and initiate joint military exercises.
This is undoubtedly the most resolved and militant posture yet adopted by President Obama. The White House has thrown its weight behind a S. Korean decision which N. Korea promised would lead to war.
The U.S. has thus called N. Korea's bluff. The next play goes to China, which must consider how to vote on a potential Security Council resolution to punish N. Korea. Thus far, every player at the table has upped the ante during his respective turn - and no one has flinched yet.
Seems the North doesn't want to wait its turn. I assume calmer minds will simply ignore this fist-pounding, but such likely-baseless claims and temper-tantrum posturing is revealing of the school-yard-bully-mentality governing North Korea.
UPDATE 2: North Korea has severed all ties with South Korea, closing off the country's air and water ways, expelling South Koreans from "a joint industrial complex just inside the North where about 120 South Korean companies employ about 40,000 North Koreans," and "totally abrogat[ing] the agreement on nonaggression between the north and the south and completely halt[ing] the inter-Korean cooperation."
Abrogating the agreement on nonaggression. I believe that makes the third time this week North Korea has declared war on South Korea.