Complex sentence: Cheyenne-American relations had been governed by the Treaty of Fort Laramie since 1851; however, American expansion into the Great Plains continued apace, especially after the Pike's Peak Gold Rush began in 1859.
Split into simpler sentences: Cheyenne-American relations had been governed by the Treaty of Fort Laramie since 1851. However, American expansion into the Great Plains continued apace, especially after the Pike's Peak Gold Rush beginning in 1859.