Since 1945, North Korea (officially the Democratic People's Republic of Korea or DPRK) and South Korea (Republic of Korea) have been in official conflict. Aggression between the two Koreas has come in ebbs and flows, though rarely has it amounted to an act such as this. The symbolic act of destroying the liasion office, a hub for building inter-Korean relations, is reported to be a result of North Korea's view of South Korea becoming the United States' lackey. As American influence in South Korea increases, North Korea has become subject once again to economic sanctions related to denuclearisation of the country.
the rest of the world should not be preparing for all-out war
However, the rest of the world should not be preparing for all-out war – at least not yet. North Korea is often portrayed as an aggressor and military threat, though this is only really true towards its neighbour. Otherwise, North Korea is a decidedly hermitic country. Historically, North Korea has in fact been an extremely peaceful country in regards to its foreign policy. It conducts itself by the national ideology of Juche ('Self-reliance') which promotes independence, non-interference, and its own sovereignity.
As far as attacks on foreign nations go, North Korea is highly unlikely to go for somewhere like the US or the UK. In the midst of economic sanctions, the coronavirus, and failures of US-North Korean-South Korean negotiations, North Korea – an already insecure nation – is under plenty of stress. This makes it even more unlikely that North Korea would wish to start war.
The real threat comes from the US, a country well-accustomed to starting war on small and non-threatening countries
The real threat comes from the US, a country well-accustomed to starting war on small and non-threatening countries. At that point, it may be more likely for the UK to be pulled through military alliances. It's a waiting game, to see how South Korea wishes to retaliate.