Basically, you've got Fail-Safe, Dr. Strangelove, The Spy Who Loved Me, Red Storm Rising, and Operation Barbarossa
Fail-Safe - a technical fault in the systems of deterrence, possibly caused by an unintended consequence of the actions of the other side, leads to war. So radioelectronic jamming triggers the Bomb/Don't Bomb computer on board some B-52s, or a flock of geese are interpreted as missiles by DEW radar system.
Dr. Strangelove - a patriot who is a few french fries short of a Happy Meal gets into a position of starting the war by falsifying orders to otherwise loyal subordinates. Col. Ripper activates Wing Attack Plan R for Robert as though DC had been decapitated in a BotB attack.
The Spy Who Loved Me - a third party pushes the superpowers towards war intending to be the last man standing after everybody else is irradiated and dead. Stromberg captures SSBNs from the US and USSR and intends each to launch on the other country, after which both will counterlaunch.
Red Storm Rising - a natural disaster leads to a resource shortage which the governing clique decides is a casus belli - "the oil is running out, so we need to invade Iran, which the Americans won't let us do, so we'll invade Germany and beat them first, and then we'll conquer Iran afterwards, and then we'll have oil, and then we'll get Eddie Van Halen on lead guitar and Wyld Stallyns will be a bodacious band!"
Barbarossa - a patriot who is a few french fries short of a Happy Meal gets made head of the government, and comedy ensues.