In 1917, Germany announced it was instituting a policy of unrestricted submarine warfare.

CNN: Saturday,

Outraged, yes, but it took two years and German resumption of unrestricted submarine warfare in early 1917 (which Mr Roberts strangely postpones to that September) to bring America to war surely the longest reaction-time in the history of galvanism.

ECONOMIST: The English-speaking world

But within six months, Germany's resumption of unrestricted submarine warfare and meddling in Mexico led Wilson to send American troops to fight in Europe -- an enormous departure from the U.S. tradition of avoiding conflicts outside the Western Hemisphere.

CNN: Obama can still build 2nd term legacy