U.S.-North Korean relations entered a new chapter following the Sept. 11, 2001, terrorist attacks on the World Trade Center towers and the Pentagon. President George W. Bush faced unprecedented challenges on how to proceed with delicate foreign policy decisions.

In his State of the Union address in January 2002, four months after the attacks, President Bush targeted North Korea as a regime that sponsored terrorism and said North Korea, Iraq and Iran constituted an "axis of evil."

"States like these, and their terrorist allies, constitute an axis of evil, arming to threaten the peace of the world. By seeking weapons of mass destruction, these regimes pose a grave and growing danger," the president explained. "In any of these cases, the price of indifference would be catastrophic."

North Korea reacted angrily to the U.S. president's remarks. In the week following the State of the Union, the official Korean Central News Agency quoted an unnamed North Korean Foreign Ministry spokesman who said the speech was "little short of declaring a war against the DPRK."

The heightened tension resulted in several dramatic incidents, including a military encounter on March 2, 2003 in which four North Korean jet fighters intercepted a U.S. Air Force reconnaissance plane in international airspace over the Sea of Japan and "shadowed" it for 22 minutes before letting it go, according to Pentagon officials who called the action a serious provocation.

The incident was the first hostile act by North Korean aircraft against a U.S. plane since the 1960s.

U.S. relations with the reclusive Communist nation further soured between 2003 and 2006 when diplomatic efforts aimed at encouraging North Korea to end its nuclear program failed and North Korea conducted its first confirmed nuclear test.

-- Compiled by Maureen Hoch for the Online NewsHour