The first paragraph and parts of the second paragraph is an actual good attempt and
start to a story and introduction to your character and sets the reasoning behind your character's motive. Paragraph three has some capitalization issues, which isn’t a big deal as long as you have a decent grasp of English, we’re not asking you to be a scholar or poet but people and the council do like to see some effort in stories.
“The CO had the MP's detained by Dennis' Squad and the CMP was trialed and executed by firing squad.”
OK, I’m assuming your squad in the story executed the MP team but you could expand upon this. What kind of guns did the execution squad use against the MPs? You could have done something ironic here like having the marines use their own sidearms against them. This is an interesting initial part of the story to expand upon, showing how and what your character feels about disloyalty and betrayal as a CO. This is something you will deal with, there will be people SUCH as ROs and SLs who say they will do something and either do it half assed either due to the fact they don’t know what they are doing or they can’t be bothered to.
Skipping forward a bit to paragraphs nine - eleven.
“ 9.But the unfortunate strikes next... During a routine patrol, the USS Franklin is boarded, by the CLF and an intense firefight begins onboard... CLF Reinforcements arrive and outnumber the Marines onboard the ship. The Captain orders an evacuation with all Military Information onboard to be wiped from the Computers, complying with orders Dennis wipes all system information on USCMC operations and informs the Captain it is time to leave. “
“10. The Captain refuses... He activates the Self Destruct and tells Dennis to get to an escape pod. Dennis nods in return and salutes for the final time. The Captain hands Dennis his ID card and an official statement of events leading to the Evacuation and Self Destruct of the vessel. “
“11. Dennis did as instructed, looking through the glass panel of his escape pod door, to see the USS Franklin explode on itself, getting into his cryo bay, he wakes up a week later in a Deep Space Station, he faxes the information given to him. “
Back to the main part of your story. Why would an expensive patrol ship with overwatch capabilities and computers with valuable, possibly unrecoverable intel need to self-destruct against a CLF threat. You could go into this easier than you may think, such as the ship carrying a small albeit important research wing with biological weapons and data, experimental combat drugs that high command
prooooooooobbably wouldn't want the public to know about, or very valuable/high-tech military equipment that could not be scuttled in a timely manner for whatever reason.
There’s also the whole cliche in your story being “started from enlisted and became Captain" as mentioned by Novacti. The whole story starts to become even worse after paragraph thirteen.