World War I broke out in July 1914, pitting the Central Powers (Germany, Austria-Hungary, the Ottoman Empire, and later Bulgaria) against the Allied Powers (Britain, France, Russia, Serbia, and several other countries). The war fell into a long stalemate after the Allied Powers halted the German advance at the September 1914 First Battle of the Marne. Leaders on both sides rejected offers by Wilson and Houses to mediate an end the conflict. From 1914 until early 1917, Wilson's primary foreign policy objective was to keep the United States out of the war in Europe and to broker a peace agreement. He insisted that all government actions be neutral, stating that the U.S. "must be impartial in thought as well as in action, must put a curb upon our sentiments as well as upon every transaction that might be construed as a preference of one party to the struggle before another." The United States sought to trade with both the Allied Powers and the Central Powers, but the British imposed a blockade of Germany. To appease the U.S., Britain agreed to continue purchasing certain major American commodities such as cotton at pre-war prices, and in the event an American merchant vessel was caught with contraband, the Royal Navy was under orders to buy the entire cargo and release the vessel. Wilson passively accepted this situation.