Complex sentence: From 1819 the Adams-Onís Treaty set the Arkansas as part of the frontier between the United States and Spanish Mexico, which it remained until the annexation of Texas and Mexican-American War in 1846.
Split into simpler sentences: From 1819 the Adams-Onís Treaty set the Arkansas as part of the frontier between the United States and Spanish Mexico. This continued until the US annexation of Texas and Mexican-American War in 1846.