I understand the original vision of starfleet and I know some hate the idea of starfleet marines or infantry or whatever.

But one thing puzzles me, if I am not mistaken in 2409 starfleet is fighting a war on multiple fronts against a lot of factions that specialize in infantry warfare. Klingons are pretty much fighters from the time they learn to walk. The borg are pretty much perfect infantry units, not to mention trouble makes like Hirogen etc. So without a solidified infantry, shouldn't starfleet be losing, or at least on the ropes getting face smashed? I mean one of those factions would have to wise up and see this gaping hole in starfleet's strategy and battles won't stay in space all the time sooner or later they will make landfall.

I don't know maybe I am over thinking.

Most, if not all, Starfleet personnel are trained in personal combat. So it's not like they'd be defenseless. But at the same time, most are also not very good at it.... most. Then you have guys like O'brien who will kick your butt with things made from scrap metal....

Starfleet does have "security" forces at the very least that are capable of fighting ground warfare. And we know from movies like Insurrection that the Federation is very capable of fielding well equipped infantry.

I see tale tell signs that cryptic thought about this as well to some extent in-game. Which is why we see phaser mini-guns, phaser shotguns, and rapid deployable turrets. But I still think starfleet shouldn't be coming out of this war as clean as they have. I would have expected to see alot more destroyed colonies, ground warzones, and alot more crippled ships in various drydocks.

Would the current STO timeline wars with the Klingons, Romulans, Borg and Undine create the necessity for these ground-forces again?

I'd make that past tense. That's pretty much the reason Omega Force exists.

Well as markhawkman stated starfleet can field a ground forces capable of infantry warfare. And in STO they kinda already reinstated the MACO (assuming it was decommissioned to begin with). But I think that if starfleet intends on coming out of this war not assimilated, not conquered or destroyed then more infantry trained forces are needed.

My characters back story deals with a similar subject.

I'm kind of new to the game and Star Trek lore. What is Omega Force?

It's a joint battle group with the KDF and Starfleet (kinda a temporary cease fire thing) that deals with overwhelming mutual threats like the Borg or Iconian's.

I would think the Federation does have some sort of ability to conduct Starship Troopers style multi-divisional ops at least in theory. There are some times where you must, repeat must have men and women on the ground to secure territory and/or restore order.

Even science missions in alien territory need attendant security detachments.

The reason why we don't see big land battles or independent land armies is because with the (proposed) space-dominance centric aspect of 23rd century warfare and beyond, and with 'modern' transporter technology it is possible to conduct proper effects-based operations (EBO) using small and highly mobile strike teams to immobilize any ground threat.

Also, photon torpedoes and other spaceborne weapons can and have been used for strikes on planetary surfaces. In a typical ground based dispute of today, no one will mess with you if you have a Galaxy class starship in geostationary position above the hot spot. If you can use starship sensors to precision target transporters to beam people and materiel into confined spaces, you can use the same awesome sensors to attack with phasers, guide photon torpedoes like Gulf War GBUs, or even beam in a photon warhead to demolish bunkers from within.

Conclusion:

As huge as a Federation cruiser is, its security detachment is probably large enough to wage a small war on its lonesome, especially with the mothership in direct support above, hence throughout the Federation's recent history a standing ground army was probably not required.

Even "infantry warfare" units as mentioned in DS9 appear to me like naval personnel detached from their ships, just like crack Japanese Navy landing forces (SNLF - Special Naval Landing Forces) in WW2 which specialized in amphibious assaults and operated under Imperial Navy control.

-The MACOS form ENT
-The ground forces from ST: TFF
-The Force O'Brien belonged to.
-And the ground forces in DS9

Marines, or volunteers or an army, or related to the Federation or Starfleet is really up to the viewer, no mention is really made in the shows (Except for the ENT MACOS).