<P> U.S. troops began arriving on the Western Front in large numbers in 1918 . </P> <P> 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 . The wider sea approaches to Britain and France, their distance from German harbours and the smaller size of the German surface fleet all made it harder for Germany to reciprocate . Instead, Germany used submarines to lie in wait for, and then sink, merchant ships heading for enemy ports . </P> <P> The United States insisted on maintaining the traditional rights of ships registered in neutral countries and protested strongly against American ships being intercepted or sunk: the British seized American ships for supposed violations, while the Germans sank them--often without warning, in violation of international law that said sailors must be allowed an opportunity to reach their lifeboats . After several violations, Germany stopped this practice but in early 1917 she decided to resume unrestricted submarine warfare, in the hope that this would starve out the British before the Americans could make any effective military retaliation . </P> <P> The British Royal Navy successfully stopped the shipment of most war supplies and food to Germany . Neutral American ships that tried to trade with Germany were seized or turned back by the Royal Navy who viewed such trade as in direct conflict with the Allies' war efforts . The strangulation came about very slowly, because Germany and its allies controlled extensive farmlands and raw materials . It was eventually successful because Germany and Austria - Hungary had decimated their agricultural production by taking so many farmers into their armies . By 1918, German cities were on the verge of starvation; the front - line soldiers were on short rations and were running out of essential supplies . </P>

Causes of american entry into world war 1