The story of the westward movement of population in the United States is, in the main, the story of the expansion of American agriculture—of the development of new areas for the raising of livestock and the cultivation of wheat, corn, tobacco, and cotton. After 1815 improved transportation enabled more and more western farmers to escape a self-sufficient way of life and enter a national market economy. During periods when commodity prices were high, the rate of westward migration increased spectacularly. "Old America seemed to be breaking up and moving westward," observed an English visitor in 1817,during the first great wave of migration. Emigration to the West reached a peak in the 1830's. Whereas in 1810 only a seventh of the American people lived west of the Appalachian Mountains, by 1840 more than a third lived there.
Why were these hundreds of thousands of settlers—most of them farmers, some of them artisans—drawn away from the cleared fields and established cities and villages of the East? Certain characteristics of American society help to explain this remarkable migration. The European ancestors of some Americans had for centuries lived rooted to the same village or piece of land until some religious, political, or economic crisis uprooted them and drove them across the Atlantic. Many of those who experienced this sharp break thereafter lacked the ties that had bound them and their ancestors to a single place. Moreover, European society was relatively stratified; occupation and social status were inherited. In American society, however, the class structure was less rigid; some people changed occupations easily and believed it was their duty to improve their social and economic position. As a result, many Americans were an inveterately restless, rootless, and ambitious people. Therefore, these social traits helped to produce the nomadic and daring settlers who kept pushing westward beyond the fringes of settlement. In addition, there were other immigrants who migrated west in search of new homes, material success, and better lives.
The West had plenty of attractions: the alluvial river bottoms, the fecund soils of the rolling forest lands, the black loams of the prairies were tempting to New England farmers working their rocky, sterile land and to southeastern farmers plagued with soil depletion and erosion. In 1820 under a new land law, a farm could be bought for $100. The continued proliferation of banks made it easier for those without cash to negotiate loans in paper money. Western Farmers borrowed with the confident expectation that the expanding economy would keep farm prices high, thus making it easy to repay loans when they fell due.
Transportation was becoming less of a problem for those who wished to move west and for those who hand farm surpluses to send to market. Prior to 1815, western farmers who did not live on navigable waterways were connected to them only by dirt roads and mountain trails. Livestock could be driven across the mountains, but the cost of transporting bulky grains in this fashion was several times greater than their value in eastern markets. The first step toward an improvement of western transportation was the construction of turnpikes. These roads made possible a reduction in transportation costs and thus stimulated the commercialization of agriculture along their routes.
Two other developments presaged the end of the era of turnpikes and started a transportation revolution that resulted in increased regional specialization and the growth of a national market economy. First came the steamboat; although flatboats and keelboats continued to be important until the 1850’s steamboats eventually superseded all other craft in the carrying of passengers and freight. Steamboats were not only faster but also transported upriver freight for about one tenth of what it had previously cost on hand-propelled keelboats. Next came the Erie Canal, an enormous project in its day, spanning about 350 miles. After the canal went into operation, the cost per mile of transporting a ton of freight from Buffalo to New York City declined from nearly 20 cents to less than 1 cent. Eventually, the western states diverted much of their produce from the rivers to the Erie Canal, a shorter route to eastern markets.

美国西进运动的故事大体说来其实就是美国农业扩张的故事，也就是一场开辟用于饲养家畜以及种植小麦、玉米、烟草和棉花的新土地的运动。1815年之后，交通的改善使得越来越多的西部农民摆脱了自给自足的生活方式，进入了国家市场经济。在商品价格较高的那些年，西迁的比率飞速增长。一名英国游客于1817年评价道：“看来旧美国正在瓦解，并移向西部”，当时正值第一次迁移浪潮。西进运动在1830年达到了顶峰。1810年的时候，还仅有七分之一的美国人生活在阿巴拉契亚山的西侧，到1840年的时候这个数字超过了三分之一。

为什么这几十万的移民——大部分是农民，还有些是工匠——会离开东部开垦好的土地和建设好的城镇？美国社会的某些特征有助于解释这场声势浩大的移民。一些美国人的欧洲祖先几个世纪以来都扎根于同一个村庄或者同一片土地，直到宗教、政治或者经济危机才迫使他们离开故土，穿越大西洋。很多经历过这场巨变的人此后都失去了把他们祖先束缚在一个地方的纽带。而且，欧洲社会相对阶层化，职业和社会地位是世袭的。而在美国社会，等级结构没有这么严格，一部分人轻易就换了工作，并且他们相信提高社会和经济地位是他们的职责。这就导致很多美国人骨子里就是不安于现状、无根而且野心勃勃的人。因此这些社会特征有利于造就出那些会冲破居住地边缘向西行进的游牧民和勇敢的移民。此外，还有一些移民迁到西部是为了找寻新的家园，获得物质上的成功，过上更好的生活。

西部吸引人的地方很多：冲积河床、绵延起伏的林地下的肥沃的土壤、大草原上的黑土，这些都吸引着在布满岩石又贫瘠的土地上劳作的新英格兰农民和饱受土壤损耗和流失困扰的东南部农民。根据1820年的一部新土地法，100美元就可以买一个农场。银行的不断发展使得那些没有现金的人贷款变得更容易了。西部的农民在贷款的时候都满怀信心，他们预期经济的发展会使农场的价格节节攀升，因此到期时要偿还贷款就比较容易。

对于那些想要迁往西部的人以及手里有多余的农产品可以供给市场的人来讲，交通也已经不成问题。1815年前，那些没有生活在通航的水路旁的西部农民只能从土路和山道去往市场。可以用家畜翻越大山，但是以这种方式运输谷物的成本是这些谷物在东部市场上的价值的好几倍。改善西部交通的第一步就是修建收费高速公路。这些公路使得运输成本有降低的可能，并且因此刺激了沿途农业的商品化。

还有两个发展预示着高速公路时代的终结，并引发了一场运输革命，使得生产日益地区专业化，国家市场经济持续增长。第一个是蒸汽船，虽然在1850年蒸汽船最终取代了所有其它的船来运输乘客和货物之前，平底船和龙骨船一直是相当重要的交通工具。蒸汽船不仅快，而且向上游运输货物的成本约是先前用手划龙骨船的十分之一。第二个就是伊利运河，它在当时是一项庞大的工程，跨越了约350英里。运河投入运营后，从布法罗（“水牛城”）向纽约运输一吨货物的成本从每英里20美分下降到了不到1美分。最终，西部各州都将不少农产品从以往的水路转到了伊利运河这条通往东部市场的捷径上来。