When did the United States declare war on the Central Powers?

The United States officially declared war with Germany on April 2, 1917, but did not declare war against Austria until December 7, 1917.

This was ostensibly in response to the sinking of the Lusitania in 1916, but really that was more of a pretense. Americans were at that time quite wary of joining a European war, and needed a push to convince them that the war was our responsibility.

But in fact the entrance of the US into the war was probably inevitable. The US had already been providing supplies---including munitions---to the UK and France since the start of the war in 1914; indeed most historians believe that the Lusitania itself contained munitions and was therefore in that sense a legitimate military target. It did have civilian passengers, but this makes them more like human shields, rather than the Lusitania just being an innocent ship attacked by the bloodthirsty Germans.