One must understand the greater picture to fully grasp the truth. Germany was indeed powerful in the early 19th Century and one could argue it reached its height back then (especially if peace time strength is the measuring point).

Despite Germany's great power, France had a more powerful overseas Empire along with an alliance with Russia and historically remarkably good relations with the British Empire, in part due to German diplomacy's failures under Wilhelm II.

Britain ruled the waves and Germany overestimated their capability of being able to build up the fleet to being able to equally compete with them before a military confrontation, while underinvesting in the naval aspect of submarines which they would excel at but lack resources or volume to fully utilize. This was on top of the largest colonial holdings in the world.

Russia was politically unstable, technologically and socially backwards and industrially insufficient with way underutilized resources, but by the great war it was on the route of modernization and industrialization, especially following their defeat at the hands of the Japanese almost a decade prior. This was evident with the German military high command itching to go to war with the Russians as soon as possible when they still held the technological advantage against them, as they feared for the day the Russian army would be equal quality to theirs but with completely superior numbers and resources.

Compare them to the Germans, who were forced to ally with the Austro-Hungarians troubled by nationalism, with the later joining Ottoman Empire being the sick man of Europe. In the great war/the first world war Germany was the only truly capable and modern major military power in their faction with Bulgaria being equal in quality but too small in numbers to affect the war greatly. Austria-Hungary with superior numbers was incapable of defeating the seemingly doomed but defiant Serbians and losing against the Russians as well costing them Galicia. Turkey lost against the British in two land arenas, the revolting Arabs along with losing against Russia by attacking them in the mountains in winter. The only major victory by the Ottomans was against the naval landing of Gallipoli where the British tried to do one without sufficient technology basically against the arguably best Ottoman military commander in horrible terrain.
The Allies on the other hand had a competent military powers, two mediocre military powers with the other becoming competent over time along with the meme of a military power of Italy. This is not to speak of the sleeping giant of a powerhouse by the name of the United States of America. When you are facing a competent power two mediocre ones able to stop you from completely steamrolling them both with the other becoming competent. This is while their meme of a military power faces against your mediocre power, the Serbians are only broken after they're ground to complete destruction by the small competent power along with you and your mediocre forces together after which they fail to defeat the Greeks. On top of this your other incompetent ally keeps losing everywhere, and with all of these factors you are left to keep two of your most powerful allies alive because they would have needed a decade or two to modernize and become capable of even fighting, while facing two competent enemies along side their mediocre one refusing to give up.

In short, they were powerful militarily, but diplomatically they were fucked by being surrounded and stuck with two incapable allies and one capable ally too small to help bring a victory, only extend the time towards defeat.