Complex sentence: The Methodists were active participants in the Oregon boundary dispute, and were part of sending three petitions to United States Congress requesting that the United States extend its jurisdiction over the portion of Oregon Country south of the Columbia River.
Split into simpler sentences: The Methodists were active participants in the Oregon boundary dispute. Members of mission were part of sending three petitions to United States Congress requesting that the United States extend its jurisdiction over the Pacific Northwest south of the Columbia River.