Several momentous events overlapped in the first months of 1917. Germany initiated unlimited submarine warfare in early February, a policy that led directly to U.S. entry into the war. The declaration of war by the U.S. Congress on April 6, 1917, brought a train of momentous events, starting with the speedy dispatch of American naval units to British waters to help fight the submarines. The U.S. government immediately seized German merchant vessels in American ports, making them available for use against Germany; it also dispatched an advance party of senior army commanders to France to prepare for the arrival of a massive American army. In the midst of this striking development, revolution broke out in Russia: bread riots in the capital at Petrograd in early March quickly led to a breakdown in military discipline in the city's army garrison, massive street demonstrations, and the collapse of the monarchy. Meanwhile, the French prepared for an ambitious advance on the western front: the Nivelle offensive. The Nivelle offensive of April 1917 quickly produced catastrophe. Nivelle based his plan on the view that the French army had to have a great victory to maintain its morale. His attack had the opposite effect; its visible failure led to massive mutinies that crippled the entire army for months to come. The French were successful in keeping the news from the Germans. As Petain's biographer Stephen Ryan has put it, "At no point in the course of World War I was the military effort of the Allies in greater danger."20 Only the restorative policies of the new French commander, Philippe Retain, starting with better food, leave policies, and medical care, then culminating in a moratorium on costly offensives, allowed the army to recover.