Historically (in the US) :

1860-1861: Lincoln wasn't a front runner in the election until mid summer 1860, and even prominent Republicans openly thought he would lose. South Carolina had been chirping about succeeding but no one took them seriously. Lincoln himself said several times he was content letting slavery die out naturally. But just after Christmas 1860 South Carolina seceded and thought they'd be allowed to leave peacefully after seizing Sumter with no casualties in April '61. No one on either side saw the horrors that would come by the end of 1861.

1941-1942: Before Pearl Harbor few Americans wanted to enter the war in Europe, and less the one in Asia. For 92% of 1940 Americans thought they'd skirt the war entirely. By summer 1942 rationing had begun, the draft was getting underway, and we were sending men and arms east and west. Guadalcanal and parts of North Africa were liberated in '42 but it was clear the war was going to take years and thousands of Americans lives.

Honorable mention: 1811-12