While one military empire in Europe lay in ruins, another awakened in North America. During the Peninsular War (1808-1814) the Spanish launched an unprecedented guerrilla insurgency undermining Napoleon's grip on that state and ultimately hastening the destruction of the French Army in Europe. The advent of this novel "system" of warfare ushered in an era of military studies on the use of unconventional strategies in military campaigns and changed the modern rules of war.

A generation later during the Mexican-American War (1846-1848), Winfield Scott and Henry Halleck used the knowledge from the Peninsular War to implement an innovative counterinsurgency program designed to conciliate Mexicans living in areas controlled by the U.S. Army, which set the standard informing a growing international consensus on the proper conduct for occupation.

In this first transnational history of the Mexican-American War, historian Benjamin J. Swenson chronicles the emergence of guerrilla warfare in the Atlantic World. He demonstrates how the Napoleonic War in Spain informed the U.S. Army's 1847 campaign in the heart of Mexico, romantic perceptions of the war among both Americans and Mexicans, the disparate resistance to invasion and occupation, foreign influence on the war from monarchists intent on bringing Mexico back into the European orbit, and the danger of disastrous imperial overreach exemplified by the French in Spain.