At the Academy, he followed the Operations track, originally specializing in Engineering, which he took for one semester, then transferred to Security and Tactical Operations.

After graduating the Academy, he served as an Assistant Tactical Officer on the U.S.S. Condor for two years. While on the Condor, he was instrumental in solving an incident at the colony on Fremoria. The colonists were being attacked by an indigenous quadripedal race. Analyzing the threat, Krol realized that the colony was built on the only area of the planet that would support the race's peculiar den habits. He brought this to the attention of the colony director, who had the colony moved immediately, dismantling the old one. Krol received a commendation for finding the best solution with the least possible casualties for both sides.

This was also the cause of his promotion to Lieutenant (junior grade) and transfer into Special Forces, and eventual posting on the U.S.S. Sunfire. During his time aboard the Sunfire, he rose to the rank of Lieutenant Commander. When the Sunfire was dry-docked for a refit, he was transferred to the U.S.S. Heisenberg.

He had only been on the Heisenberg a short time when, on an away mission, his team of three was trapped in a cave-in. One of the team, an ensign, was severely wounded and near death. Communicators were ineffective due to a freak ion storm in the upper atmosphere, and there wasn't enough air in the cave for the team to wait it out. With the other team member, a lieutenant in the medical field, tending the wounded ensign, Krol personally dug through 30 metric tons of rock to free them from the cave. Upon their return to the ship, the ensign was taken into surgery, but suddenly went into a cardiac arrest and died before the procedure could begin. Krol received an award for heroism and was promoted to the rank of commander. He still felt that the death of the ensign was a blemish upon his honor, though, and has strived to regain that honor by going even more above and beyond.

Once promoted to commander, he was transferred to the U.S.S. Washington to serve as Executive Officer.

A surprisingly short time after being put on the Washington, a command became available to him: the U.S.S. Potomac. She was an old Ambassador-class ship, and had seen her share of wear and tear, but Krol accepted command eagerly. Not too long after he assumed command of the Potomac, the ship gained sentience, and was destroyed while holding the crew hostage. After the destruction of the Potomac, the crew was rescued by the U.S.S. Kriemhild and taken to Starbase 47, where they were given their new ship, the U.S.S. Minerva.

Krol served as Commanding Officer of the U.S.S. Minerva for a few years, exploring the galaxy along with his crew and sometimes meeting great adversity. The Minerva was instrumental in the Federation efforts during the Dominion War, and it was during the war that one of the greatest adversities struck.

During an attack run on the Dominion-held world of Hiramma, the Minerva was caught by surprise by several Jem'Hadar warships and severely damaged. By the end of the battle, despite completing the mission's objective, the ship was nearly crippled, and had to be towed in to Starbase 47 for repairs. Nearly half of the crew either died or were presumed dead in that mission, including Krol's executive officer and love of his life, Be'Karlsa, who led an away team planetside for a ground-based strike.

In a continuation of bittersweet victories for Krol, he was promoted to the Admiralty for the success of that mission and its contribution to the success of the Federation effort in the war. At the time of the promotion, he felt no real joy at the honor and would have taken a leave of absence if not for the importance of his new position.

A few months after his promotion, the Dominion War ended, and Krol finally took a leave of absence to go back to Qo’noS and grieve for Be’Karlsa. While on Qo’noS, he heard a rumor through an anonymous source that Be’Karlsa and a number of other members of the away team she had led were still being held prisoner by the Dominion on a secret base in the Gamma Quadrant: a clear violation of the terms of the peace treaty. Krol brought his concerns to Starfleet Command, but was unable to substantiate them enough for them to take official action. Any chance of Be’Karlsa still being alive was enough for Krol, so he decided to take “unofficial action.”

After returning to active duty at Starbase 47, he commandeered the Minerva and took her through the Bajoran Wormhole, setting a course for the coordinates specified by his source. The asteroid was right where it was supposed to be, and Krol was able to rescue all Federation citizens from the prison. The resulting battle during the Minerva’s run back to the Wormhole nearly destroyed the ship, and once she returned to the Alpha Quadrant, she limped slowly to Deep Space Nine.

For his actions, Krol was court martialed, and at the trial it was decided that in this case the ends justified the means enough to keep from discharging Krol completely from Starfleet, but he was demoted to the rank of captain and relieved of duty for six months. At the end of those six months, repairs to the Minerva were just being finalized, so Krol was returned to command of the ship he so loved.