The plot, basically:

The World Government has declared war against the current ruling Yonkou after Akainu's sudden announcement that all pirates having bounties over 100million beli should be rounded up and executed publicly. Some speculates that the announcement was a propaganda thought up by the Tenryubito to finally seize total power over the world, but others suspects that an even greater force was behind all of this. Although the idea seem absurd at first, however, with the not so private amassing of Pacifitas and powerful devils fruit users, the Government has been successful so far, executing over 150 pirates with bounties ranging from 200 to 700 million belly in plain public. Threatened by the turn of events, the Yonkou was forced to form a temporary alliance to strike back. The collision of both powers has tipped the world balanced immensely, and thus started the most anarchic era in history.