From Halopedia, the Halo wiki
The Purple Heart is a United Nations Space Command decoration, awarded to UNSC troops who have been injured or killed by enemy attack in combat situations with enemies of the UNSC.
Originally conceived by the military leadership of the United States of America on Earth in 1932, the Purple Heart is still awarded in the UNSC over 600 years after its conception. For the most part, the medal's appearance has not changed in this time. The only difference between the UNSC's medal and the original decoration is that the current medal lacks the white stripes on the edge of the ribbon. The medal even bears the same profile of George Washington as the original, paying homage to the man who first created a medal for troops wounded in battle for their country.
The UNSC Purple Heart is awarded to UNSC troops who have been injured or killed by enemy attack in combat situations with enemies of the UNSC. This may include combat with Insurrectionists or the Covenant. The Purple Heart has been awarded to huge numbers of UNSC troops, as a great many have been injured fighting the Covenant.