The German use of unrestricted submarine warfare was a clear breach of international law in force at the time. The American declaration of war was largely a response to this breach. It came after repeated diplomatic pressure on Germany failed to prevent it continuing to use the tactic.

The main piece of international law that controlled commerce warfare during WWI was the Hague Convention of 1907. This prohibited sinking of merchant ships without warning. Ships had to be stopped before they could be sunk, and their crews and passengers placed in a safe place before the ship was sunk. Said safe place did not include lifeboats unless the ship was close to shore. Neutrals could only be sunk if they had been searched, and that search had revealed contraband. Unrestricted submarine warfare ignored all of these rules. Ships were sunk without warning, which meant that their crews and passengers were placed at risk. Neutrals were sunk indiscriminately, without ensuring that they were legitimate targets. The Hague rules were difficult for subs to follow, but were necessary for the safety of neutrals - a submarine captain could not tell the difference between a ship carrying ammunition and one carrying puppies to Belgian war orphans through his periscope.

In February 1915, the Germans first introduced unrestricted submarine warfare as a tactic. The orders that implemented it called for subs to only attack passenger and cargo ships of belligerent powers, but clearly accepted the risk of sinking neutrals. On the 7th May, U-20 sank the Lusitania. While the Lusitania was a British ship, and carrying contraband, the sinking was still in clear breach of the Hague Convention. The passengers and crew had been given no warning of the attack, no opportunity to abandon ship or evacuate. 1,195 civilians, 123 of whom were Americans, were killed. This drew a strong protest from the Americans, who focused heavily on the breach of international law. The initial German response to the protest brushed off the issues, putting blame on the British through their measures to protect ships against submarine attack. The American response restated their initial position, demanding that the Germans apologise and pay compensation. The German response to this was somewhat sarcastic, likely due to a need to play to public opinion at home, but contained a promise to protect American ships and civilians. The American counteroffer again repeated the need for a German apology and reparations, but also included two concessions. In essence, these amounted to an offer to force the British to weaken their blockade of Germany if the Germans gave up unrestricted submarine warfare.

There was no German response to this offer. Negotiations within the German government were ongoing but were overtaken by events. On the 19th August, U-24 sank the British liner Arabic, outbound from Liverpool, without warning. This sinking killed 47 civilians, two of whom were Americans. The initial German response to this sinking was unrepentant. They refused to apologise or pay indemnities. While they were willing to argue the case in the Hague arbitration courts, this was only under the condition that the judgement would not have any wider effects on the policy of unrestricted submarine warfare. A later note, after American threats to break off diplomatic relations, saw Germany agree to pay an indemnity, but without accepting that such an indemnity was required. It also stopped short of abandoning unrestricted submarine warfare, instead stating that submarine captains had been ordered to take more stringent steps to identify targets. In February 1916, the Germans accepted liability for the Lusitania sinking and again claimed that it had imposed restrictions on its submarine force. However this was undermined by the fact that, eight days earlier, they had declared that any armed enemy ship was open to attack without warning. On the 23rd March, the unarmed French liner Sussex was attacked. While Sussex remained afloat, 50 civilians were killed. This made a mockery of the German promises. The US, now aware that the Germans had been lying to them all along, threatened war. In response, the Germans compromised, imposing restrictions on the submarine force.

In 1917, though, the Germans returned to unrestricted submarine warfare. The pressure of the British blockade and the failure to win naval supremacy in the North Sea at Jutland meant that this was the only way the German Navy could contribute to victory - starving Britain before Germany could starve. On the 1st of February 1917, unrestricted submarine warfare began. On the 3rd, the American steamer Housatonic was sunk by U-53. While this sinking followed the Hague rules, it came on the same day that the Americans broke diplomatic relations with Germany over the resumption of unrestricted submarine warfare. On the 7th, U-85 sank the British liner California without warning, killing 41 civilians. On the 25th, the liner Laconia was sunk, again without warning. This killed two American civilians, pushing America further towards war. Beyond the Housatonic, other American ships sunk before American entry to the war included the Lyman M Law, the Algonquin, the City of Memphis, the Illinois, the Vigilancia and the Aztec. Some of these ships were carrying contraband or items that could be considered contraband, but others, like the City of Memphis, were empty. The Vigilancia and Algonquin were carrying non-contraband cargoes. These sinkings made it clear to the US that the Germans had no interest in sticking to their promises of 1916 and holding to international law. The Zimmermann Telegram made this clear. It showed the US that Germany was aware it was in breach of international law, and that its actions were likely to drive it into conflict with the USA. While it was only hypothetical, it was still an outrageous position to take.