Odds and Ends
Switzerland accidentally invaded Liechtenstein... More than
23 5, 2:47am
We never crossed the border into China during the Korean war, and it was actually mostly a fairly easy to follow border since it was largely defined by two rivers and a mountain. This was well known by the UN forces. Macarthur never intended to invade China (the general you mean) but he did essentially ignore orders not to push the North Korean troops all the way too the border. The plan was to defeat them and allow a buffer zone to exist between Korea and China, and if any troops were to go up to the border they could only be Korean troops.
Macarthur ignored this and pushed to the border, which did exactly what the UN leaders were worried it would....bring China full scale into the war. And they pushed us back to the old borders between the two Koreas at the start of the war. Leading to the stalemate of today.
There might not be a North Korea today if he had listened to orders.
I've read a little more on it, and the traditional view was that the Chinese thought they'd be invaded (and Manchuria over the border was an important industrial center), but recent scholarship thinks they knew they wouldn't be attacked....BUT did NOT like the idea of a US allied Korea bordering them. Perhaps fearing future invasion rather than present invasion. North Korea served Chinese interests as a buffer state....pretty much up until recently even. China now hates North Korea lol.
However you're right about Vietnam. And those were no accidents. The North Vietnamese forces were freely crossing the borders as well, helping a Communist revolution in Laos for instance. And the Americans also crossed and bombed Laos and Cambodia to stop North Vietnamese supply lines and troop movements and stuff. Which in their brutality helped undermine the Cambodian government at the time, accidentally helping the Khmer Rouge come to power....quite possibly the worst regime to ever exist. Worse than ISIS even.