Why is the War of 1812 commonly called the war of 1812?
The War of 1812 began in 1812.
It used to be referred to as the War of 1812-1814, but that name became too cumbersome and ultimately, through general usage, was shortened to the War of 1812.
There were at least two. The first was called either the American Revolutionary War (or just the Revolutionary War in the US), or the American War of Independence. The second is usually called the War of 1812 (sometimes the Anglo-American War of 1812, since there were several other armed conflicts going on at the same time).