I believe the answer would be 1378
Involving us in world war 2
They were kept from combat roles.
For the most part, it began because due to the war with Napoleon of France, and the issue of conscription, Britain had to "kidnap" American ships to search for their own people that escaped from conscription. Not only that, they blockaded American ships that were headed for France. These marine issues agitated the States into eventually declaring war on Britain. Although the States fought poorly and Britain basically would have won, they settled for a treaty. Americans thought they won, and called it the Second Independence or revolution. It's true, it was the second time they stood up to Britain and got away with it, so their national morale grew enormously. Especially with what Andrew Jackson did in the end, easily beating back the Britain troops who were unaware of the treaty at the time.