Basically the US declared war on the UK while we were fighting the French and invaded Canada despite Jefferson's assertion that its conquest was a mere matter of marching and the small british forces we defeated all 19 invasion attempts during that war. Try JM Hitsmans The Incredible War of 1812 about that war.
It was part of the Napoleonic wars and the British soldiers in that war called themselves the Forgotten soldiers just like the ones fighting the Japanese in Burma. Both the Japanese and the US at the time shared similar charachteristics strangely enough.
There was also the little matter of the Royal Navy impressing US sailors.
Actually they were deserters from British ships being recaputred. But the whole impressment issue was finsished before the war even began and it was really about conquering Canada and kicking the British out of North America.
Oh, I agree that the USA wanted Canada and grabbed onto the impressment issue as an excuse to start fighting.
However, many Royal Navy captians simply seized however many people they needed off US ships, regardless of nationality and/or prior RN service That is what the captian of H.M.S. Leopard did and was the spark that began the war.
Also, the US position at the time was that any person serving on a US-flagged ship was a US citizen, regardless of country of origin so no other country (including Britian) had any claim on a US citizen. This protection even extended to RN deserters.