When the US invaded Mexico (1846-48) to seize much of its territory, the US troops burned Catholic churches, raped Mexican women, and desecrated the Blessed Sacrament. They also taunted the small Catholic minority in the US army, most of whom were Irish and German immigrants, and who were horrified at what the US army was doing. In response, many of them left and joined the attacked side. The "San Patricios" were called traitors by the US, and many were executed after the US won the war. Of course, the Mexicans saw them ina different light. Here is a not-all-bad Wikipedia article.