When supposedly asked what a Prime Minister feared most, Harold Macmillan may have said, "Events, dear boy, events." [I felt compelled to put in all those qualifiers because Saint Google reports that the British prime minister may never have actually said those words.] Spurious or not, the point is still valid. Actual events, especially unexpected ones, can derail even the most carefully planned government policies.
On the other hand, events can also provide opportunities for action, and for policy change. The fiscal cliff came about because of action-forcing processes that didn't work out as planned. Deadlines can be action-forcing when they are perceived as real and unavoidable. Mass killings in schools also create opportunities to discuss and maybe act on measures to reduce gun violence. Catastrophic storms force people and governments to rethink many of their policies.
On this 150th anniversary of the Emancipation Proclamation, it is useful to remember that Lincoln waited to announce his intention until just after the Union victory at Antietam. Two years later, he was reelected in part because, just a few weeks before the vote, General Sherman had captured and burned Atlanta. Those events created a new and better context for the subsequent actions.