The Soviet Union and China almost destroy each other via Cracked.

via Cracked...(there are 4 other stories of "almost" wars so check it out).

Being the two predominant communist superpowers, the Soviet Union and Mao Zedong's China were technically fighting on the same side during the Cold War. In reality, though, the two countries had constant disputes, accusing one another of overreacting to shit and getting into arguments over territory -- hell, they were basically a married couple that only stays together because they are bonded by their mutual hatred of Sam from next door.

By the late 1960s, arguments escalated to the point of silverware-throwing. The Soviets had denied Mao aid to build an atomic bomb, possibly because he was unnervingly comfortable with the concept of nuclear war. Denied his toys and frustrated because the Soviets were ignoring his political views, Mao wanted some goddamn respect. So in 1969, the Chinese military invaded Soviet-occupied Zhenbao Island, which Mao claimed was originally Chinese territory. Things proceeded as you'd assume: Shots were fired, soldiers got killed, both sides moved more troops to the region, and shit got real.
Ownership of the island tipped back and forth, and things got more and more hostile. It looked like the two powers were set to rip each other apart before the baffled democracies they were supposed to be fighting had a chance to make some popcorn.
What Stopped It:
Mao backed the hell down.
When war fever-gripped both the Chinese and Soviet people, Mao realized that challenging an opponent that had a strong track record of kicking dictator ass and a bunch of nuclear weaponry might not be the most solid strategy, even if China had its own nuclear program up and running at that point. Meanwhile, the Soviet leaders were totally cool with a throwdown, calmly checking with the bemused Americans if theywould be OK with the Soviet Union pre-emptively nuking the shit out of China.
If the War Had Happened:
Two of the biggest armies at the time would have slaughtered each other, with or without nukes.
A Soviet war with China would have involved two incredibly large conventional armies getting into an epic slugfest. Knowing that both parties had access to tactical nuclear weapons and were led by some fairly ... volatile people, it would not have been impossible that either country would have also used tactical nukes to tip a bloody conventional war in their favor.

I love Cracked.  Not because its precise, but because its a quick, simple overview of events.  Probably one of the best ways I've seen to get the uninterested, interested in history.