The main plank in the Free Soil Party's platform was, of course, opposition to the spread of slavery into the western territories. Essentially, then, they supported the so-called Wilmot Proviso, which would not have allowed slavery to spread into lands gained during the war with Mexico. The Free Soilers were, in many ways, a single issue party, and this was their major issue. However, they also embraced a number of probusiness measures, such as a modest protective tariff, investment in internal improvements, and inducements to settle the frontier, including the rudiments of what would become the Homestead Act. What should be noted is that the Free Soilers did not generally call for abolition (though some abolitionists joined their ranks.) Rather, they argued for restrictions on the expansion of slavery, which they regarded as inimical to the interests of white workers. As historian Eric Foner has observed in his important book on Free Soil ideology, the Free Soilers' political beliefs amounted to "a justification of antebellum northern society" which entailed, necessarily, a critique of southern society, especially its foundations in slavery.