Growing discontent with the New Order eventually coalesced into the Alliance to Restore the Republic, which began a military campaign against Imperial forces, that became known as the Galactic Civil War. A reorganized command structure, eventually consisting of twelve Grand Admirals, supervised the rapid expansion of the fleet that had begun during the Clone Wars. The Imperial Navy was formed from the Republic Navy of the Galactic Republic in 19 BBY, as part of the newly-proclaimed Emperor Palpatine's Imperialization campaign during the rise of the Empire. At its peak, it fielded millions of warships, logistics vessels and fighters, becoming one of the largest navies in galactic history and maintaining Imperial control throughout the galaxy. The Imperial Navy, also known as the Imperial Starfleet, Imperial Fleet, Imperial Battlefleet, Imperial Space Navy, or Imperial Space Service, and informally referred to as ' the Empire's long arm,' was the space warfare branch of the Imperial Military of the Galactic Empire. We who serve the Emperor in the Imperial Navy have made this Empire the irresistible, galaxy-spanning force it is today.' ―Fleet Admiral Holt, cadet indoctrination speech excerpt

Only the best, the most disciplined, are admitted to our ranks. The might of the Empire rests on the military - and the Imperial Navy embodies the most elite and trusted segment of that power. ' … You are part of the greatest navy ever assembled.

Star Wars canon ranks of Imperial Navy