> An obvious one was at the time of WWII. If the clever people had refused to work on weapons, things would have finished up potentially a lot worse for mankind generally.
There is still debate about whether the use of nukes was at all justifiable, a lot of the motivation for researchers was to stop the Nazis from getting there first, not beating Japan into submission.
That said, this isn't WW2. The US has the largest military on Earth and the most advanced weapon systems. US drone strikes are killing targets in civilian areas without repercussions. The POTUS has the authorisation to wage war against whomever he chooses as long as he can somehow relate them to someone who was involved in 9/11. Heck, the invasion of Iraq even violated international law with no repercussions.
The US military (CIA included) can pretty much do what it pleases and kill people whenever and wherever it wants. Don't want to kill right now? Off to Guantanamo they go. If they happen to be a US citizen just say they're an enemy combatant and make sure they never see a domestic court of law. Even torture is permissible.
If you're worried the US might not be sufficiently equipped to become a dictatorial global authority, you haven't been paying attention.