The American Civil War was a cornerstone event for the Irish in America. Their participation proved their duty to Union and Confederate causes, increased their acceptance in American society, and hastened assimilation. While the Irish participation and hard fighting reputation in the Civil War improved Irish acceptance in society, it proved ephemeral. However, it did provide the Irish with a new confidence and a sense of self-determination. The significant impact of the war was its unifying ability within the Irish communities. The war caused the Irish to organize and create a supporting infrastructure. This infrastructure and organization advanced the Irish in postwar America. The Irish became dominant in urban labor organizations, Democratic Party politics, and city government. Furthermore, this organization allowed the Irish to dominate later immigrant groups, during the height of Great Atlantic Migration. “Ni neart go cur le cheile--Togetherness is Strength,” is the bond that made the Irish overcome discrimination, adversity and war, and succeed in postwar America. Applications of this study can be used to assess other immigrant groups’ acceptance in America, both past and present, and as an applied model for their foreign-born military service.