WWI began neutral for the US but trade with Allied powers like Britain compromised neutrality. Germany began attacking civilian ships, killing Americans on the Lusitania in 1915 and resuming unrestricted submarine warfare in 1917. The Zimmerman Telegram revealed a German plot against the US, prompting Wilson to declare war in 1917. The US mobilized over 2 million men for the American Expeditionary Force under General Pershing. Though inexperienced, the AEF helped break the Western Front stalemate in 1918 by capturing over 132 German prisoners in the Meuse-Argonne Offensive. Germany surrendered in late 1918, ending US involvement in WWI after over 320,000 American casualties.