Final answer:
Reconstruction efforts were undermined by the political discord between Andrew Johnson and Congressional Republicans, the economic devastation of the South post-war, and violent opposition from groups like the Ku Klux Klan striving to maintain white supremacy.
Step-by-step explanation:
Three major factors that undermined U.S. Reconstruction efforts in former Confederate states following the Civil War included political conflict between President Andrew Johnson and Congressional Republicans, the economic devastation of the South, and the emergence of white supremacist groups such as the Ku Klux Klan.
Firstly, political conflict after 1865 between President Johnson and Congressional Republicans led to inconsistent Reconstruction policies. Johnson sought a swift reunion favoring non-slaveholders and marginalized former slaveholders and slaves, while Republicans aimed to punish former slaveholders and advocate for Republican support through coalition-building between yeomen and freedmen.
Secondly, the South's economy was crippled, with widespread destruction and a labor system in disarray following the abolition of slavery. This economic destitution resulted in perpetual poverty and the rise of sharecropping, which hindered economic recovery and social progress.
Lastly, violent backlash from white supremacist groups, especially the Ku Klux Klan, actively resisted Reconstruction efforts. They aimed to maintain pre-Civil War racial hierarchies, intimidating and assaulting Black people and their allies, and severely impeding social reform and integration.