What specific incident brought the US to declare war on Germany?

There was no specific incident unless you consider Germany declaring war on the USA first the incident. Tensions had built for years. Bluntly, the USA was much more antagonistic towards Germany than Germany towards the USA. There is the infamous 'peacetime' sinking of a German Uboat by a USA destroyer and the subsequent machine gunning of helpless survivors in the water - an incident, had it been reversed, would probably have intiated war. Of course there was also the arming and supplying of all of Germany's enemies during time of war - the so called 'lend lease' program. LL was sold the USA public as like lending a hose to a neighbor who's house is on fire but in reality it was an enourmous give away of tax payer money to Britain and Russia. President Roosevelt was trying to get the USA involved in the European war against Germany but had been unable to rouse American public support for his endeavor. Germany finished the debate by declaring war obstensibly to help Japan after Pearl Harbor.