Near the end of Night, Elie Wiesel realizes that the lines of battle are approaching Buchenwald. The SS prepares one last effort at resistance:
The battle did not last long. Toward noon everything was quiet again. The SS had fled and the resistance had taken charge of the running of the camp.
At about six o’clock in the evening, the first American tank stood at the gates of Buchenwald.
Elie both throws himself into the provisions (they have had nothing but grass to eat for almost a week) and thus doesn't focus on the reaction of the Americans, so we have to look elsewhere for that perspective.
The soldiers described the scene as "hell." Horse stables designed for 50 horses were crammed with 2000 people. Corpses littered the ground. Prisoners were covered in infectious sores and crude bandages that looked like toilet paper. The prisoners looked at their liberators with a vacant stare that seemed unable to comprehend this turn of events. The soldiers noted that the people no longer looked human due to their severe starvation and abuse. Americans turned to the nearly dozen imprisoned and surviving physicians to help treat these former prisoners and to understand the medical history they had accumulated under Nazi rule.
These images and efforts to help such decimated people forever changed many of the American soldiers who liberated the camp. Some were unable to ever speak about what they had seen to anyone. The visual horror of how humans can destroy other humans was simply too overwhelming, and some tried to bury these mental images after leaving the camp.
Note: The link below provides incredible firsthand knowledge of the liberation from the American perspective, but because of the nature of the topic, there are some disturbing images. Please consider this before using the link as an additional resource.