American ships became targets of attack by 1917 because

American ships became targets of attack by 1917 because Americans continued to trade with the warring nations.

At that time, the american vowed that we adopt a non-interventionist policy during the war time. But when we continue to trade with warring nations, other nations viewed it as we supplying materials and supplies to one of the nations that involved in the war. This is why american ships became targets of attack.