The Joint POW/MIA Accounting Command, a Defense Department Agency, is charged with searching for, recovering, and identifying Americans from past conflicts, from WWII through the end of the Cold War.

For this time period, they are not assumed dead until their remains are recovered. That said, their family members have received benefits. But they are searched for to this day.

That really depends on the nation involved.

I do know what happened to Soviet MIA's and its rather sad.

A little background information, the Soviets at the time, used somethign that we wouldnt really recognize as dog-tags. They used a small vial of ebony which contained a blank piece of paper that soldiers were supposed to fill out with their information. Now, most of these often went un-filled because the average soldier was superstitious... They believed that if they filled them out, they were signing their death warrants.

So, when someone went missing in action, the Soviets often assumed that the soldier had defected or deserted. They often didn't retrieve their dead in the early war because they couldn't or it would have been far too dangerous to try.

After the war, the Soviet leadership decided that it was in their interest to get rid of anything war-related that did not fit the official narrative. This included the tremendous losses that the Soviet Union suffered to hold on against the German onslaught . They recovered the dead that they could and started planting trees over major battlefields or building things over places where there'd be a lot of dead Soviet soldiers... Russia, today, still has 4 million WW2 MIAs and it has led to volunteer organizations going out into the woods and digging for the dead themselves.

To know more, here's a nice documentary from the BBC for you, check it out I can't link directly to it but its called "Digging up the dead in Russia" Feb. 13th 2014.