Unrestricted submarine warfare.
Germany was warned during WWI that the US would break off relations if the unrestricted submarine warfare continued. They choose to continue this type of warfare and Wilson carried through.
This type of warfare used by Germany meant that any boat, enemy or not, would be shot down if a German U-boat could do so. This policy prevented the US and Great Britain from trading, making business leaders in the US upset. Also, passenger boats had been targeted such as the Lusitania.
