lord of the rings - What are the "Powers" quoted by Gandalf to Frodo in "The Shadows of the Past"?

The "Powers" referred to in this passage are most likely the "movers and shakers" of Middle-earth; the leaders, nations, consuls and individuals who most affect the lives of the people. This would include the more powerful nations, like Gondor, Rohan, Lothlórien, etc, as well as groups like the Wise, and influential individuals like Saruman, Gandalf, Elrond, Círdan, etc. And of course, it includes the biggest power in the world that even the isolated Hobbits have heard about, Sauron. It might also include the Valar, though by this time they had withdrawn from the affairs of Middle-earth (including physically removing the Undying Lands from Middle-earth) and their only obvious involvement was in sending the Wizards.

If Gandalf had said in the 1940's, "But there is only one Power in this world that knows all about nuclear weapons and their effects", he would be saying that of all the powers of the world (Germany, the British Empire, USSR, Japan, the League of Nations, etc), only one power, the USA, understands nukes.