Hi Ewan.
Well my knowledge of history is very limited but:
. USSR and China
I missed that, surely if they had been going at it hammer and tongs I would have heard about it two almightly great states like that. Anyone have dates or a link or something? If China & USSR were at significant war with each other then that kills the question (any communist countries at war with each other?)
. China and Vietnam
Was that not China coming to Communist North Vietnam's aid against South Vietnam and the USA?
. Vietnam and Cambodia
I know nothing of this conflict. Will try to look it up.
. USSR and Hungary.
. USSR and Czechoslovakia
Squatticus says: "the USSR was aiding those countries against subversive elements! You might as well add between USSR and socialist Afghanistan!"
Hi, sorry was typing off the top of my head and couldn't remeber the details, but here they are!
USSR and China, see
http://en.wikipedia.org/wiki/Sino-Soviet_split.
Not an all out war but border clashes in 1969. It may not have been an all out war but is incredibly important as it almost became one and because some of the subsequent wars between Communist countries are based on the Sino-Soviet split.
Vietnam and Cambodia was in 1979
http://en.wikipedia.org/wiki/Cambodian-Vietnamese_War. The Cambodian Communist party had followed the Chinese and Maoist groups whereas the Vietnamese had sided with the USSR following the Sino-Soviet split. (It's very complicated and is as much to do with ethnic tensions between the Cambodians and the Vietnamese, hundreds of years of history of tension between the two societies and the geopolitical interests of the two countries at the time as it was to do with political differences). Cambodia had been provoking the Vietnamese since the Khmer Rouge had come to power with minor border skirmishes and incursions. In 1979 Vietnam was fed up of this and launched a full on invasion of Cambodia, chucked the Khmer Rouge out of power and occupied the country for several years whilst fighting against the remnants of the Khmer Rouge. (In one of the more bizarre examples of realpolitik during the Cold War the remnants of the Khmer Rouge were supported by the US and the west whilst fighting against the Vietnamese).
Vietnam and China was also in 1979,
http://en.wikipedia.org/wiki/Sino_vietnam_war.
As I said earlier, the Vietnamese sided withe USSR following the Sino-Soviet split and there were increasing tensions between the China and Vietnam. (In short, China was very big and very powerful and the Chinese Communist party had been very influential in the early stages of the Vietnamese Communist party development, but tended to relegate Vietnamese interests behind those of China and the Vietnamese resented this. Similarly, the Vietnamese Communist party had dominated the early Indochina Communist Party and had relegated the interests of Cambodia behind Vietnam's, which is why the Cambodians sided with China). Following the invasion of Cambodia by Vietnam, China invaded northern Vietnam. The war only lasted for about a month but was an absolute disaster from the Chinese perspective, though they claimed it as a victory.
USSR and Czechoslovakia was in 1968, also known as the Prague Spring
http://en.wikipedia.org/wiki/Sino_vietnam_war. Basically, the Communist leader of Czechoslovakia was far more moderate and liberal than the USSR was happy with. He introduced a large number of reforms and political life became more free. The USSR opposed this and tried to negotiate an end to the reforms. These negotiations failed and the USSR (and Bulgaria, Poland, Hungary and East Germany) invaded to reverse the reforms and get rid of the reformers.
USSR and Hungary doesn't fit as a war between Communist countries, actually. I wasn't thinking when I typed!
http://en.wikipedia.org/wiki/Hungarian_Revolution_of_1956.
There was a revolution in Hungary in 1956 which overthrew the Communist government and subsequently the USSR invaded to crush the revolutionary movement and reimpose a suitable leader.