From Halopedia, the Halo wiki
It was preparing to launch a reconnaissance mission as part of Operation: HYPODERMIC in 2552, when the Covenant attacked the planet Reach. The vessel was ordered to drop its mission and return to defend the planet, where it was probably destroyed during the Fall of Reach.
The UNSC Euphrates is most likely named after the Euphrates River in Iraq. It, along with the Tigris River, formed an area in the Middle East known as the "Fertile Crescent". This area was the birthplace of several large ancient empires, including the Assyrians and Sumerians.
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