"We are in the business of assuring our South Korean allies that we will help defend them in the face of threats," Pentagon press secretary George Little said, adding, "We are looking for the temperature to be taken down on the Korean peninsula."
Even without nuclear arms, the North poses enough artillery within range of Seoul to devastate large parts of the capital before U.S. and South Korea could fully respond. The U.S. has about 28,500 troops in the South, and it could call on an array of air, ground and naval forces to reinforce the peninsula from elsewhere in Asia and the Pacific.
In just the past few months, North Korea has taken a series of steps Washington deemed provocative, including an underground nuclear test in February. In December the North Koreans launched a rocket that put a satellite into space and demonstrated mastery of some of the technologies needed to produce a long-range nuclear missile. And several weeks ago, the North threatened to pre-emptively attack the U.S.
Bruce Bennett, a specialist in North Korean affairs for the RAND Corp., said he believes much of the recent taunting from North Korea reflects turmoil among the ruling elite in Pyongyang. He cited unusually high turnover among senior officials during the 15 months that Kim Jong Un - grandson of the nation's founder - has been the top leader.
"I think with the purges going on, he's got some instability that is generally not being recognized" outside of North Korea and that may be pushing Kim to take a more confrontational stance, Bennett said in an interview. "He's trying to be blustery to make it appear that he's really in control, he's really strong and he can defeat us."
In response, the Pentagon announced it would beef up missile defenses based on the U.S. West Coast, and it highlighted over a period of days the deployment of B-52 and B-2 bombers, as well as two F-22 stealth fighters, to South Korea as part of an annual U.S.-South Korean exercise called Foal Eagle, which lasts through April.
On Tuesday, officials said the Navy was keeping the USS Decatur, a destroyer armed with missile defense systems, in the vicinity of the Korean peninsula for an unspecified period instead of continuing its journey back to the U.S. after a Mideast deployment. And they said a similar ship, the USS McCain, had been shifted slightly to the waters off the southwest coast of the Korean peninsula as a further precautionary move.
North Korea has been an enigma to most outsiders since it was founded by Kim Il Sung in 1948. The United States has often misjudged the North's political path. After the founding Kim died in 1994, for example, U.S. intelligence officials said they believed his successor son, Kim Jong Il, would be more accommodating to the West.
"Flaky as he may be, (Kim Jong Il) nevertheless ... realizes the only way they're going to extricate themselves from the shambles that their economy is in now is to get outside help," James R. Clapper Jr. told a congressional panel in January 1995. Clapper was director of the Defense Intelligence Agency at the time; today he is President Barack Obama's most senior intelligence adviser as director of national intelligence.