The Amphibians were an alliance of all those whom lived undersea, which came into existence by the actions of Namor. Deciding that Atlantis had to go to war with the surface dweller, his wife Dorma who was against the war counseled that if they were to go to war they needed to wage it on multiple fronts. Therefore Namor formed pacts with the various undersea communities of Homo mermanus, uniting them in an alliance.
They commenced their offensive on the surface world, by using leviathans which attacked surface dwellers' maritime shipping and coastal cities, bringing maritime commerce to a halt. After which the Amphibians' forces launched attacks on numerous major seaports around the world. In short order nearly every major coastal city (e.g. San Francisco and Yokohama), had been captured. Emperor Namor then informed the United Nations, that there would be no more war as long as they respected the Amphibians' rule over the oceans and the enclave ports.
The United Nations grudgingly accepted this, and though there were some uprisings in the ports they were all crushed. Occasionally the Amphibians would execute a sea or air raid to take out a center developing new weapons to use against the Amphibians. Their rule over the ports continued until the arrival of the interdimensionally-displaced Fantastic Four changed things. Resulting in the death of Namor at the hands of the traitorous Byrrah, a short-lived uprising in NY City, and an agreement for the Amphibians to eventually return the cities to the surface-dwellers while they kept their rule over the seas.