Given that the US was heavily involved in SE Asia and civil discord and anti-War sentiment in the US, what prevented the North Koreans from taking advantage of the situation and invading the South? Did the US military presence in SK intimidate them? Did China or the USSR talk them out of it? Did Kim il Sung have second thoughts given that he was nearly overthrown after his defeat in the Korean War? Did North Korea feel the South had surpassed them in military capabilities?
The North did attempt to start an insurgency in the South and staged a number of raids across the DMZ, resulting in significant upgrades to border defenses and patrols and ROKA counterinsurgency operations further inland. The capture of the Pueblo and the downing of a USAF ECM aircraft were also part of this conflict.
Neither side felt that a conventional attack was winnable given the stalemate that they had reached before the armistice, and the North did not feel it could win a protracted war against the larger South, so it attempted to start an insurgency there instead. The KPA infiltrated special forces into the Jirisan and Taebaek mountain areas, but the population they expected to convert to their cause turned them in, and the ROK responded with a heavy counterinsurgency effort that ended any chance of further operations. The US spent hundreds of millions of dollars on upgraded border fences, changing the DMZ from a warning line against conventional attacks to a tightly sealed anti-infiltration barrier.
Scenes from an Unfinished War: Low intensity conflict in Korea 1966–1969