To knock France out immediately via the Schlieffen Plan and gain additional colonies, territory, prestige. Germany didn't go about starting the war, it simply wanted to catch up to other countries in colonies, naval power, and influence. Austria-Hungary and Serbia/Russia started the war. Knocking Britain out became a later goal as they tried to sink food shipments coming from South America that fed the British. Keeping America out was an early goal which evolved into keeping America occupied long enough to win before American forces could arrive.