In the west, Germany sought a quick victory by encircling Paris using the Schlieffen Plan. But it failed due to Belgian resistance, Berlin's diversion of troops, and very stiff French resistance on the Marne, north of Paris. The Western Front became an extremely bloody battleground of trench warfare. The stalemate lasted from 1914 until early 1918, with ferocious battles that moved forces a few hundred yards at best along a line that stretched from the North Sea to the Swiss border. The British imposed a tight naval blockade in the North Sea which lasted until 1919, sharply reducing Germany's overseas access to raw materials and foodstuffs. Food scarcity became a serious problem by 1917. The United States joined with the Allies in April 1917. The entry of the United States into the war – following Germany's declaration of unrestricted submarine warfare – marked a decisive turning-point against Germany.