Following the sixth and most powerful of the Democratic People's Republic of Korea's (DPRK) nuclear tests last Sunday, US President Donald Trump again pointedly declined to rule out a possible US military response. South Korea, in response, has fully deployed its THAAD anti-missile system, despite opposition from China and Russia. But these are just the most recent events in an issue that has been simmering for decades.

Tensions on the Korean Peninsula have been lingering since the 1950s, when the Korean War broke out.

Tensions lifted slightly, in particular when the DPRK agreed to the nuclear Non-Proliferation Treaty (NPT) in 1985.

Six years later, along with South Korea, the DPRK signed the South-North Joint Declaration on the Denuclearization of the Korean Peninsula, under which both sides agreed not to use nuclear weapons in any way.

In 1992, less than a decade after signing the Non-Proliferation Treaty, the DPRK concluded a comprehensive safeguarding agreement with the International Atomic Energy Agency.

Then the country announced its withdrawal from the organization, after the director of the US Central Intelligence Agency (CIA) speculated that the DPRK may have produced one or two nuclear weapons in 1994. 

However, after Former US President George W. Bush took office in 2002, the relationship between the two countries turned particularly sour, especially after Bush labelled the DPRK as part of an "axis of evil."

In 2003, the Six-Party Talks started, aiming to end the DPRK's nuclear activity. In return, the country called for the US to sign a "non-aggression treaty" and to normalize diplomatic relations - but the Bush administration responded that a formal non-aggression pact was "off the table," despite a US promise not to attack the DPRK. The Six-Party Talks were suspended in 2007 and in 2009 Pyongyang announced it would withdraw from the negotiations completely.