No, It was the Pol Pot government as he turned into an extremist Darwinian, he took the "Survival of the fittest" motto to heart as Hitler and Stalin did.
My guess is that it is most unlikely that any formal apologies will be made.
The US did not as such; except when it comes to the Khmer Rouge claim to the Cambodian seat in the UN, after the "Killing Fields". The US supported the Khmer Rouge claim to the UN-seat, not the Vietnamese puppet-governments claim.
The US president during WW2 was Franklin D. Roosevelt.
Abraham Lincoln was president during the US Civil War.
James Monroe, the 5th president of the US was president during the Missouri Compromise.
James Knox Polk was the US president during the Mexican War.
who was the us president at the time of the battle of France
the president during ww1 was Wilson
who replaces the president of the US senate in his absences
Clinton
Woodrow Wilson was the US president during WW I.