The yellow years are ones in which the USA was getting into or out of major wars — or both, in the case of the brief Spanish-American War. The red years are war years, and the blue years are years of (relative) peace.

The sectors are each bounded by two radii, and a 1.5° arc. The current year is omitted intentionally because 2016 isn’t over yet, and we don’t know what will happen during the rest of it.