I read that book 20+ years ago and I am trying to find its title.
The story is about space marines who visit various worlds and encounter new species. One original element of the story is a space marine can adjust some settings of the suit by moving switches with his tongue.
The Forever War by Joe Haldeman had tongue controls in the powered suits. The suits were clearly and obviously inspired by Starship Troopers, but the rest of the war was more of a reaction against that vision -- and surely also a response to the Vietnam War, in which the US was still embroiled when the book came out, and in which Haldeman had served as a combat engineer.
The suits also had a number of other noticeable things -- laser weapons that fired from a fingertip, insulation so efficient that even in liquid helium temperature conditions, damage to the cooling radiator would result in cooking the occupant in a short time, and in later chapters, a coating that protected against the "stasis field" that effectively converted a far-future war into a medeival one fought with swords, spears, and darts.
Other notable points were instantaneous "collapsar" jumps for interstellar travel (none the less involving years, decades, even centuries of time dilation from the relativistic maneuvering before and after the jump), ships that maneuvered at extremely high acceleration requiring passengers and crew to ride in liquid baths, and a family name chosen by hippies who misspelled the peace sign they were trying to adopt.
As others have said; The Forever War, by Joe Haldeman.
Tongue controls (Chapter 7 and 13);
All I could hear was Potter's breathing and the soft crunch-crunch of my boots. Couldn't see much of anything, so I tongued the image converter up to a log two intensification. That made the image kind of blurry but adequately bright.
I shuffled over to my place on the perimeter to watch for god knows what... but I was so weary I couldn't keep my eyes open. Finally I tongued a stimtab, knowing I'd pay for it later.
Low Tech warfare (Chapter 34);
Inside, I could see the rocket that had missed me drifting lazily through the gloom, rising slightly as it passed through to the other side of the dome. It would vaporize the instant it came out the other side, since all of the kinetic energy it had lost in abruptly slowing down to 16.3 meters per second would come back in the form of heat.
Nine people were lying dead, facedown just inside of the field’s edge. It wasn’t unexpected, though it wasn’t the sort of thing you were supposed to tell the troops.
Their fighting suits were intact—otherwise they wouldn’t have made it this far, but sometime during the past few minutes’ rough-and-tumble, they had damaged the coating of special insulation that protected them from the stasis field.
So as soon as they entered the field, all electrical activity in their bodies ceased, which killed them instantly. Also, since no molecule in their bodies could move faster than 16.3 meters per second, they instantly froze solid, their body temperature stabilized at a cool 0.426 degrees Absolute.
I arranged the archers in a circle around the fighter (its landing fins would give them partial protection from missiles coming in from behind) and between each pair of archers put four other people: two spear-throwers, one quarterstaff, and a person armed with a battle-ax and a dozen throwing knives. This arrangement would theoretically take care of the enemy at any range, from the edge of the field to hand-to-hand combat.
... even if you're not sure, read it anyway.
It's one of the best SF books ever written, winning the Hugo, Nebula, and Locus awards for Best Novel.
Starship Troopers.
They wear the suit, and it determines its movements from the wearers muscle movements. So for things like the legs and arms, the marine doesn't think about, they just move like they normally would. The suit keeps up and augments their motions.
Some functions, such as communications, are controlled physically with the mouth, since their hands and legs will be otherwise occupied.
As already indicated, this is probably The Forever War by Joe Haldeman. However, if it is not, it might conceivably be The Languages of Pao by Jack Vance.
Per Goodreads:
The Panarch of Pao is dead and Beran Panasper, his young son and heir, must flee the planet to live and avenge his father's death. It is at the secret fortress on the planet Breakness that Beran discovers the dreaded truth behind the assassination of his father—and much more. The people of Pao are a docile lot, content to live in harmony with the rest of the cosmos, but the scientists at Breakness seek to alter the psychology of the Paonese for their own purpose—and Beran holds the key to their audacious plan. Beran will return to Pao, transforming his home world beyond his teacher's wildest dreams. But though he has been fashioned into a man of Breakness, Beran's heart is of Pao. And he brings to his world the seeds of change that will save Pao... or destroy it.
Part of the process of "being fashioned into a man of Breakness" entails being fitted with robotic enhancements to his body, which Beran controls by pressing buttons (located inside his mouth) with his tongue.
