Our ships were attacked during WWI because they were transporting arms to the British and French. The US government was trying to claim neutrality while supplying one side with military aid, and that didn't go over well with the Germans.
As for WWII, it should first be noted that if America had not intervened in WWI, WWII would have been much less likely to happen. At the very least, Hitler would never have come to power, and it's possible that the Bolsheviks would not have remained in power for long. That being said, how much was our security threatened during that war? True, the Japanese launched a fairly devastating air raid on Pearl Harbor (after being goaded, time and again, into attacking the United States), but how likely was a Japanese invasion of the US to happen, much less succeed? As for Germany, they hadn't even been able to invade Britain, and the invasion of Russia was consuming more and more of their resources--even Hitler wasn't rash enough to seek a war with America when he was having enough trouble fighting the wars he'd already started.
