A nation or empire can do it without bloodshed if both countries agree to it. It is called Annexation.
Because you could just sit in your trench and wait for the opponent to attack, at which point you could shoot him, throw explosive devices, or even use chemical weapons that weren't banned then.
Answer:
Europeans, maybe also Americans.
Explanation:
The Europeans are the most apparent. Britain and Germany were incredibly strong forces in China during and leading up to the Boxer Rebellion. America was also heavily involved. If I had to choose one, I would say Europe, but if not Europe and America were both heavily involved.
