The United States entered into World War I on 6 April 1917, more than two and a half years after the war began in Europe. Apart from an Anglophile element urging early support for the British and an anti-Tsarist element sympathizing with Germany's war against Russia, American public opinion had generally reflected a desire to stay out of the war. Over time, especially after reports of German atrocities in Belgium in 1914 and after the sinking of the RMS Lusitania in a torpedo attack by a submarine of the Imperial German Navy off the southern coast of Ireland in May 1915, Americans increasingly came to see Imperial Germany as the aggressor in Europe.

While the country was at peace, American banks made huge loans to the Entente powers (Allies), which were used mainly to buy munitions, raw materials, and food from across the Atlantic in North America from the United States and Canada. Although President Woodrow Wilson made minimal preparations for a land war before 1917, he did authorize a shipbuilding program for the United States Navy. Wilson was narrowly re-elected in 1916 on an anti-war platform.

By 1917, with Belgium and Northern France occupied by German troops, the Russian Empire experiencing turmoil and upheaval in the February revolution overthrowing the Tsar on the Eastern Front, and with the remaining Entente Allied nations low on credit, the German Empire appeared to have the upper hand in Europe. However, a British economic embargo and naval blockade were causing severe shortages of fuel and food in Germany. Berlin then decided to resume unrestricted submarine warfare. The aim was to break the trans-atlantic supply chain to Britain from other nations to the West, although the German high command realized that sinking American-flagged ships would almost certainly bring the United States into the war.

Imperial Germany also made a secret offer to help Mexico regain territories of the Mexican Cession of 1849, lost seven decades before in the Mexican–American War of 1846–1848, (now incorporated in the Southwestern United States) in an encoded diplomatic secret telegram known as the Zimmermann Telegram, which was intercepted by British intelligence. Publication in the media of that communique outraged Americans just as German submarines started sinking American merchant ships in the North Atlantic in their U-boat campaign. President Wilson then asked Congress for "a war to end all wars" that would "make the world safe for democracy", and Congress voted to declare war on Germany on April 6, 1917. US troops began to arrive in Europe later that year, and served in major combat operations on the Western Front under the command of General John J. Pershing, particularly during the final Hundred Days Offensive.

Britain used its large navy to prevent cargo vessels entering German ports, mainly by intercepting them in the North Sea between the coasts of Scotland and Norway. By the end of 1915, the Royal Navy had successfully interdicted and stopped the naval shipment of most war supplies and food to Germany. Neutral American merchant cargo ships that tried to trade with Germany were seized or turned back by the Royal Navy in outlying waters who viewed such trade as in direct conflict with the Allies' war efforts. The impact from the blockade became apparent very slowly because Germany and its allies controlled extensive farmlands and raw materials on the continent of Europe and could trade with land-bordering neutral countries like Sweden and the Netherlands who were not themselves blockaded by the British or French. However, because Germany and adjacent Central Powers ally Austria-Hungary had decimated their agricultural production by drafting and taking so many farmers and supplies of nitrate fertilisers into their armies, and the Allies were able to pressure neutral countries into reducing exports, the situation worsened, with the so-called "turnip winter" of 1916–1917 an example of the emerging severe shortages in Central Europe. The situation at the start of 1917 was such that there was clear pressure on the German leadership to avoid a "war of exhaustion", while the softening of neutral trade reduced the importance to keep the neutral countries on side.

Germany, for her part, had considered a blockade from 1914. "England wants to starve us", said Grand Admiral Alfred von Tirpitz (1849–1930), the man who built the Imperial German Navy fleet after 1871 with the unification of Germany during the last few decades and who remained a key advisor to the German Emperor / Kaiser Wilhelm II. "We can play the same game. We can bottle her up and destroy every ship that endeavors to break the blockade". Admiral Tirpitz wanted to sink or scare off merchant and passenger ships en route to Britain. He and others in the Admiralty reasoned that since the island of Britain depended on imports of food, raw materials, and manufactured goods, preventing a substantial number of ships from supplying Britain would effectively undercut its long-term ability to maintain an army on the Western Front and potentially force Britain into speedy surrender. Such strategy would also give an essential, war winning role to the German Imperial Navy, who had been mostly passive in the war thus far due to being unable to challenge the powerful Royal Navy surface warship fleet.

While Germany had ample shipyard capacity to build hundreds of U-boats, they had only nine long-range U-boats at the start of the war in August 1914. Nevertheless, without consulting colleagues earlier superiors like Tirpitz, the outgoing head of the German Admiralty Hugo von Pohl (1855–1916), declared the beginning of the first round of unrestricted submarine warfare six months after the war began in February 1915. However, instead of ceasing shipping to Britain and blaming the British (as the Germans positioned the move as a reprisal) as the Germans anticipated, the United States demanded that Germany respect the earlier peace-time international agreements upon "freedom of the seas", which protected neutral American and other ships on the high seas from seizure or sinking by either belligerent. Furthermore, Americans insisted on strict accountability for the deaths of innocent American civilians, demanding an apology, compensation and suggesting that it is grounds for a declaration of war.

While the British Royal Navy frequently violated America's neutral rights by defining contraband very broadly in their naval blockade of Germany, German submarine warfare threatened American lives. Wilson's top advisor, legendary Colonel Edward M. House (1858–1938), commented that, "The British have gone as far as they possibly could in violating neutral rights, though they have done it in the most courteous way". Further, while the British justified their argument with an appeal to precedent, the Germans claimed that they should be allowed to use their new weapon to its best potential and so existing rules and norms need not apply. This is especially exemplified when German submarines torpedoed ships without warning, causing sailors and passengers to drown. Though in practice this was initially rare, since U-boats preferred to attack on the surface, this strategy was justified by claims that submarines were so vulnerable that they dared not surface near merchant ships that might be carrying guns and which were too small to rescue submarine crews. The Americans countered that if the new weapon cannot be used while protecting civilian lives, it should not be used at all.