I'm having a hard time thinking of "information that would keep one country from launching nuclear weapons at another". Unless that information was "this country is planning a nuclear strike", and then the threat of retaliation could very well save lives.
As an example, let's say one country poisoned the leader of another country 20 years ago and covered it up. Exposing that information could start a war.
If it's the right action to leak that information is entirely dependent on your moral philosophy. Utilitarians would generally want to keep it a secret, though.
That is not entirely true. Governments, unlike individuals, are fairly rational agents and do not start wars for revenge. For all we care, the murdered president was member of the opposition and his disposal might have helped the current ruling party.
I think you are being overly generalist here. Governments are as rational as the people that make them and run them, in the same way as a corporation (or any group of people, really). Can you really say that all governments are fairly rational? How about North Korea?
At least the motive of corporations is clear: profit. A (democratically elected) government is in constant conflict with itself over ideology (a much trickier motive to grasp, one that often causes irrational behavior), and it doesn't always take consensus to act.
Could you give me an example of what you mean?