In WWI subs started with highly restricted rules, more or less similar to the ones listed in J.T. McDaniel\'s post. These were loosely known as the \'cruiser rules\', as raiding cruisers were supposed to follow them.

They were impractical for subs for the same reasons as in WWII. Of especial note is that merchant ships, especially British ones, did not folow the cruiser rules: armed with guns, radioning their position, even orders to ram U-boats, the aggressive actions of merchants made the cruiser rules impractical.

U-boats gradually moved towards unrestricted warfare. Restrictions were loosened, then tightened. Germany was caught on the fence between wanting to use a highly effective weapon against England without the very significant restrictions that the cruiser rules applied, and not wanting to anger America into entering the war. The Navy and the foreign service battled over this issue repeatedly. The Lusitannia was sunk during an early \'unrestricted\' phase (near the British isles only), and the US reaction convinced the Germans to clamp the restrictions back down.

Only in 1917, when Russia was collapsing, did Germany make a major gamble: switiching its Russian front armies to France, and trying for a decisive 1917 offensive, they would try to knock England and France out of the war. U-boat restrictions were lifted because it was felt that this would weaken England during the critical 1917 offensive. This led, as expected, to a US decleration of war, but Germany gambled that they could win before the americans could enter the war in force. The gamble came close, but the offensive failed, and Germany, broke and starving, surrendered in the fall of 1918 when it became clear that the 1919 offensive, the first at which significant US forces would be available, would break her lines and lead to defeat.

In WWII, much the same seesaw started: U-boats operated at first under heavy restrictions out of fear of angering the US into entering the war. Ocean liners were off limits, even if thought to be a troop tranport. At one point even destroyers were forbidden targets, less a US destroyer be shot by mistake. Roosevelt, unlike the passionately nuetralist Wilson of WWI, wanted the US to enter the war, as he felt democracy itself was at stake, and it was suicide to sit on the sidelines while Germany swallowed the democracies of the world one by one. The US Navy policy in the Atlantic towards U-boats was agressive, to say the least, for a neutral nation. Numerous incidents occured, including the loss of a US destroyer by accidental fire from a U-boat.

When Japan jumped, Germany declared war on the US, and the U-boat rules were all lifted. As J.T. pointed out, the US operated under unrestricted warfare from day one.

It should be pointed out that restrictions placed on sub warfare had little to do with human compasion or moral restraint, but rather the fear of upsetting powerful neutrals. With no neutrals present, the sub war was generally unrestricted. U-boats in the med in WWI, were US ships were unlikely to be found, did not play by the tight rules of those around England.

But even in \'unrestricted\' war, there were rules. Neutrals and marked hospital ships were not to be attacked, a restriction that was in genral honored by subs of all nations. It was also a \'loophole\' that was widely abused - the British flew merchant ships with US or other neutral flags, and the Japanese sailed a suspiciously large number of hospital ships that were known to be carrying munitions......still, the subs held their fire...