The most important institution in medieval Europe was the:

The Roman Catholic Church

Explanation:

After the fall of Rome, no single state or government united the people who lived on the European continent. Instead, the Catholic Church became the most powerful institution of the medieval period. Kings, queens and other leaders derived much of their power from their alliances with and protection of the Church.

the answer is

it was the "4. potsdam" conference that demanded the unconditional surrender of japan, since it was clear that the japanese would continue to find until the very last man otherwise.