U.S. Army was far too small and spread too thin to accomplish such an endeavor.

The Northern Democrats never supported the conditions of Reconstruction. Once the Republicans were divided and the Democrats able to consolidate power, Democrats were able to exert their political will and make their bargain with newly-elected President Hayes to end Reconstruction. Considering all of this, I might argue that the Democratic party was a major obstacle to the success of Reconstruction.

By far the biggest obstacle to Reconstruction was the opposition of white Southerners. Almost immediately after the war, whites in the South formed vigilante groups to terrorize freedmen and passed legislation aimed at preserving the racial order in the South. These so-called "black codes" ended with the advent of congressional Reconstruction, as radicals in Congress secured laws and eventually the Fifteenth Amendment that guaranteed suffrage to African-Americans. This was a major advance, but still in many states "redeemer" Democrats were able to seize control of legislatures as early as 1870. Throughout the South, Reconstruction was always hindered by violence and the threat of violence. Bloody "race riots" in which white mobs attacked African-Americans occurred throughout the South, most infamously in Memphis and New Orleans in 1866 and Colfax, Louisiana in 1873. Federal legislation in the form of the Ku Klux Klan Acts helped to limit the activities of terrorist organizations, but they proved difficult to enforce in the long run, and white racism and violence proved to be a major force in returning white Democratic governments to power throughout the South.

Perhaps the other most important factor hindering Reconstruction was the unwillingness of most in the federal government to enact land reform. Aside from a few isolated experiments like the "40 acres and a mule" promise in the Sea Islands of Georgia, neither the US Army nor the federal government showed any stomach for confiscating the lands of slaveholders and redistributing them to the people who had worked them. Rather, the federal government, through the Freedmen's Bureau, sought to broker labor contracts between freedmen and landowners. Over time these contracts became sharecropper arrangements, and this system mired most African-American farmers in debt and poverty. Even most Radicals believed only in "free labor," and they held property rights, even of former rebels, to be sacrosanct when it came to land redistribution. As a result, African-Americans gained limited political freedoms, but the vast majority lacked economic security.