Complex sentence: Fort Steilacoom was founded by the U.S. Army in 1849 near Lake Steilacoom and was the first military fortification north of the Columbia River in what was to become Washington state.
Split into simpler sentences: Fort Steilacoom was founded by the U.S. Army in 1849 near Lake Steilacoom. It was among the first first military fortifications built by the U.S. north of the Columbia River in what was to become Washington state.