In 1803, an unanticipated opportunity presented itself when France offered to sell its territory west of the Mississippi River to the United States. President Jefferson set aside his usually cautious exercise of federal power and quickly took up the offer. He soon launched four expeditions into the prairie and mountains to explore this huge acquisition of land. The powerful Osage of the Arkansas River valley responded to overtures for an alliance and were soon lavishly welcomed by Jefferson in Washington City, but the even more powerful Comanche of the southern Great Plains stood their ground against all invaders. Meanwhile, the expedition by Lewis and Clark, the longest and northernmost trek of the four launched by Jefferson, mapped U.S. terrain all the way to the Pacific Ocean, giving a boost to expansionist aspirations.