Between Planets by Robert A. Heinlein

Don Harvey was attending school on Earth when his parents suddenly and urgently called him home to Mars. He had been skeptical about the talk of interplanetary war breaking out if Mars and Venus followed through on their threats to declare independence from Earth, but he was wrong. War broke out, and he was stuck on Venus, with no way of getting home.

Then there was the ring that an old family friend had given him just before he had left Earth. Shortly afterward, the friend had been questioned by Earth's secret police and had died--from "heart failure," they claimed. When Earth troops landed on Venus and started looking for Don and that mysterious ring, he realized that he was trapped in the center of a war between worlds that could change the fate of the Solar System forever

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