From the souvenir book of the 15th IVU World Vegetarian Congress, India, 1957:

Emperor of Persia, who conquered many lands and raised Persia from an obscure country into a great Empire was not only vegetarian himself but ordered his soldiers also strictly to adhere to this diet. His army conquered wherever it went, Xenophen says that Cyrus was brought up upon a diet of bread, cresses, and water till the age of fifteen, when honey and raisins were added.

From the World convention on reverence for all life , Pune, India from 22nd to 24th of November 1997:

An interesting point mentioned by Dr. Rooyintan Peer was that Cyrus the Great and his army were Vegetarians.

These comments are uncorroborated - the speaker in the 2nd item may well have used the first as his source...

According to Wikipedia on Cyrus : "Cyrus was born in either 590 or 576 BC. . . . After his father's death in 559 BC, Cyrus became king of Anshan. . . . Cuneiform evidence from Babylon proves that Cyrus died in August 530 BC" - this goes on to talk about Cyrus as a great humanitarian, freeing slaves, and an inspiration to the founding fathers of the USA, but no mention of vegetarianism.