War With Mexico

The War of 1812 demonstrated that despite its geographic and political isolation from Europe, the United States still needed to maintain military forces. The militia component of that military force was increasingly filled by the growing number of volunteers (as opposed to mandatory enrollment) militia. Many states began to rely completely on their volunteer units, and to spend their limited federal funds entirely on them.

Even in the mostly rural South, these units tended to be an urban phenomenon. Clerks and craftsmen made up most of the force; the officers, usually elected by the members of the unit, were often wealthier men such as lawyers or bankers. As increasing numbers of immigrants began to arrive in the 1840s and 1850s, ethnic units such as the "Irish Jasper Greens" and the German "Steuben Guards" began to spring up.

Militia units made up 70% of the U.S. Army that fought the Mexican War in 1846 and 1847. During this first American war fought entirely on foreign soil, there was considerable friction between regular Army officers and militia volunteers, a friction that would reappear during subsequent wars. "Regulars" were upset when militia officers outranked them and at times complained that the volunteer troops were sloppy and poorly disciplined.

But complaints about the militia's fighting abilities declined as they helped win critical battles. The Mexican War set a military pattern which the nation would follow for the next 100 years: the regular officers provided military know-how and leadership; citizen-soldiers provided the bulk of the fighting troops