Reasons for breakthrough
The settlement could be attributed to the flexibility of both sides, especially the US backing off demands that North Korea disclose its uranium enrichment and proliferation activities as part of the declaration.
Washington has taken a pragmatic stance: rather than let the declaration issue stall the disablement of Yongbyon nuclear facilities started under the October 2007 agreement, it'd better to make a deal to continue to dismantle already-known nuclear abilities.
North Korea also made well-meaning gestures, including handing over some 18,000 pages of its Yongbyon nuclear logs to the US to qualify its statements on nuclear program.
The breakthrough made over the declaration was meaningful to North Korea. Denuclearization was set to be accomplished in line with the principle of "commitment for commitment, action for action".
The US will accordingly start the process of removing North Korea from a list of state sponsors of terrorism and stop penalizing it under the US Trading with the Enemy Act.
If North Korea was really de-listed and sanctions against it were lifted, the US-North Korea bilateral ties would be improved further, which would also help North Korea going back into the international community and create a more advantageous international environment for its development.
The improvement of US-North Korea ties will also help ease the tensions on the Korean Peninsula and is conducive to safeguarding peace and stability in the region.