A German Western European empire in 1915 would still face a long war

Could Britain, Russia, and the United States defeat Germany if it controlled much of Western Europe by June 1915?

Yes, but only through a long and painful war in which secure British ports, resistance in occupied territory, and Russian and American resources eroded Germany's initial advantage.

The scenario begins with a short-term German triumph followed by severe strategic overextension. British ports around the Irish Sea would remain beyond easy German attack and continue supporting the war. Occupied France, Belgium, and the northern countries would require garrisons and invite guerrilla resistance. Russia and the United States would add enormous manpower and industrial depth once fully mobilized, while aircraft production and development could accelerate in response to the new geography. Germany might look dominant in 1915, but holding the conquests while defeating those remaining powers would make the war longer and increasingly costly.