How did congress make certain that southern states followed the laws of reconstruction

by force they sayed that they were going to bring the military

Congress sent the military to the South to rule the Southern states and ensure that they obeyed the law.

Explanation:

The Reconstruction Era started with President Lincoln and after his assassination, President Johnson took over. Johnson did not have the same beliefs as Lincoln had, he believed that the Southern States should have more liberty and because of that, he vetoed a series of laws passed by Congress. With that, the Congress restricted the powers of the President and took over Reconstruction in a period known as Congressional Reconstruction.

During this period, Congress passed the Military Reconstruction Acts of 1867 that divided the south into five military districts that were placed under military leadership.