World War I
A British citizen can join the US Army, and a British soldier might be attached temporarily to a US unit, or sent to a US training/instructional programme, but they won't be able to be in both services simultaneously.
Pontiac wanted to join forces with the Native Americans because Pontiac saw the British as a threat and wanted to fight the British with the Native Americans.
In 1899, may 30th.
The French Forces in Britain was very small. Not that many made it out of France after the Battle of Dunquerqe (Dunkirk). The French armed forces did join with Britain and the US forces and they were known as the allied forces.
American forces
Join the US Armed Forces.
Because they were promised freedom.
US militias operating on the Western frontier and the borderline with British Canada thought that Canadians would rust to join forces with the US and be a part of the US republic. To the surprise of the Americans, Canadians, including the French settlers in Quebec, wanted to remain loyal to the British Crown.
It didn't the US was formed when it revolted against the British Empire
Enslaved people were seeking liberty
I would say that if you're Canadian, you should support your own troops by joining the Canadian military, and if you're British you should join the British Forces.
Not in the US military, no.