Americans traditionally linked western migration with the preservation of opportunity. The revolutions in markets and transportation greatly accelerated the process of white settlement beyond the Appalachian Mountains. Similarly, the spread of cotton production and the quest for economic success carried southerners, white and black, free and slave, westward. Thus, white southerners linked western expansion with the preservation of not only white opportunity but also slavery. As the lines of settlement spread across the Mississippi River into the Louisiana Purchase, Americans increasingly cast their eyes on neighboring lands to the west that seemed ripe for acquisition. They also increasingly responded to a longstanding sense of mission, which envisioned the United States as a beacon for the world, and to the widespread belief in the impending millennium that fostered so many reform movements in this period. Economic, political, and intellectual forces combined to produce a new expansionist surge in the 1840s that pushed the Republic's boundaries to the Pacific, a process that brought Americans into contact with other cultures and peoples.