Finally, in 2235, the Federation Council authorized the creation of a Starfleet Marine Corps. Commodore Kripney transferred his commission to the newly created corps and was promoted to Major General, becoming the first commandant of the Marine Corps.

The Starfleet Marines became a magnet, attracting many of the finest volunteers from the armies of Terra and Andor, both planets with long military traditions. In 2236, the Marine Corps Academy opened in Syrtis City on Mars to provide the fledgling Corps with its first specially trained officers, and in so doing reignited a friendly inter-service rivalry that goes back to 18th century Terra.

The value of the new Marine Corps was quickly demonstrated during the Four Years War. In one of the first engagements involving the marines, the officer corps of the 4th marine regiment was killed when the USS Landover was destroyed in orbit over Joia V on 10 November 2240. Sgt. Maj. Randall Robor-Wheeler, the senior NCO present, assumed command of the regiment and lead it in a campaign of guerrilla resistance and harassment of the Klingon army of occupation until relieved over six months later. Sgt. Maj. Wheeler was promoted to captain and given command of the 1st company of the 4th regiment.

In January of 2241, the 2nd, 8th and 13th marine regiments were dispatched to Sinbad IV to defend that world from an imminent Klingon invasion. With the numerically superior Klingon invasion fleet keeping the Starfleet ships busy, 12 Klingon regiments were landed on Sinbad IV. This not only marked the first large scale ground action involving the Starfleet Marines but also the first test of the first graduating class from the Marine Academy. Though greatly outnumbered, the Starfleet Marines showed incredible valor and resourcefulness. With the timely assistance of Starfleet artillery, the marines captured or killed over 30,000 Klingon soldiers and won a stunning victory.

In August 2241, the marines spearheaded the first Federation assaults into Klingon space. The 5th Marine regiment lead the landings on Rudgur III. This operation saw the first use of chlortheragen gas against the Starfleet Marines. Losses were high and the remnants of the 5th regiment were evacuated after sustaining extreme casualties.

Perhaps the most famous marine exploit of the Four Years War was the assault on Karag II on 5 February 2243, when the 1st, 3rd, 7th, 12th and 22nd marine regiments were landed on Karag II. Atmospheric conditions made orbital support impossible and the marines took on the Imperial Army in numerous engagements, even conducting house to house and hand to hand assaults. On 12 February, the weather broke and Starfleet's orbital superiority was brought to bear and the Marines secured the planet. Due to the many acts of extreme valor performed during the Karag campaign, the Federation Council created a new decoration, the Karagite Order of Heroism and Colonel Ahab Anchar was the first recipient.