Last night was creating another fighter character, the image Portrait I used was a Solider from the old Union Army. Still working on the backstory as well as debating on other military personal(s) but I was wandering how would a person who has died on the front lines (Be they Army Ranger, NAVY, Marine, special unit/Forces, ext) fair if they were Isekaid in the fantasy setting? How would they feel about the other races, how would there weapons work against others and how many would feel about them?

How well would they face off against each race one on one both unarmed and armed?

If your character met one of them out in the woods frantic and going gun blazing after being ported in a world like this how would they react and what would they do?

There's no one possible answer for any of those questions.  Every different possible answer is equally correct.

Those are questions that you are meant to be exploring as you develop your character and play the game. We could offer our opinions, but any character based on those answers would be our characters, and not yours. We'd be robbing you of some of the most important and enjoyable parts of the game. The parts where you enter character and start viewing the world through their eyes.

Agree with 6thLyranGuard - this is going to be up to how you want to play the character and how your DM constructs their world and what species they put in it.  

To build off that, this is really something you need to work with your DM on, not other folks on the internet.  There are a number of problems to having a character who gets isekaid into the game--most notably it is blending genera in a way that might not be to the other players'/DM's liking, that player is going to have knowledge everyone else in the world might not that could break immersion or give them a leg up, and it breaks a bit of the fantasy escapism if you are bringing in real world history, conflicts, and politics into the game.  That is not to say it cannot work out--I am sure there are plenty of groups who would like this kind of character--just you should not assume it will be a welcome addition to the party and need to talk to the DM and other players instead of strangers on a forum. 

Just thought to get some input on it. I was just curious is all and it had my brain itching in back of my mind.

Well, I can tell you how I would handle it:

From the DM’s perspective, I would probably say no—I am an incredibly liberal DM in terms of backstory, but bringing the real world into the game is where I would draw the line.  I would possibly suggest that the player instead be isekaid from a different fantasy world, such as being brought from Forgotten Realms into a homebrew world. That provides the fish out of water perspective (though I think there are better ways to create that), but lacks the real world baggage. 

If I were to allow it, the person would be just like any other human in terms of strength and abilities - nothing special or abnormal for the world.  I absolutely would avoid DMing a John Carter situation, where a civil war vet shows up and is just better than all the others in the isikai world.

From a player perspective, I would likely play the character as surprised, confused, and a bit frightened, especially at first given the jarring move from a battle to a new world. Over time, that would dissipate as they get more used to their new reality. 

Oh hell no, trust me I’m not trying to make him Hacksaw Ridge or the Super Soldier. I would have them confused beyond their minds possibly a lot of stats in Constitution so they won’t freaking out, too much, after seeing new species like a orc, bugbear, or a dragon.

A high constitution means they won't get the runs after drinking the local water.  It won't do anything to help them process suddenly seeing  a fantasy creature for the first time.