Reality check, is reality check, and what better answer than reality?
There have been occurrences of monkeys picking up knifes, for instance:
Drunk monkey attacks bar patrons with knife (watch video)
Or this one:
Monkey Using Knife To Carve Jack-O-Lantern (watch video)
As Ghotir mentions, Javkie the baboon participated in WWI:
Until August 1915, Jackie was the beloved pet of the Marr family, who lived on Cheshire Farm, Villieria, on the outskirts of Pretoria. When, as No 4927, Private Albert Marr attested at Potchefstroom on August 25 1915, for service in the newly-formed 3rd (Transvaal) Regiment of the 1st South African Infantry Brigade, he asked for and was given permission to bring Jackie along with him. (...) He drilled and marched with his company and would entertain the men – such entertainment would become all important to relieve the boredom of the stalemate of trench warfare once the Brigade reached France.
At night when on guard duty with Albert, he was particularly useful because of his keen eyesight and acute hearing. He could give early warning of enemy movement or impending attacks with a series of short, sharp barks and tuggings at Pte Marr`s tunic. Jackie wore his uniform with panache, would light up a cigarette or pipe for a pal and always saluted an officer passing on his rounds. He would stand at ease when requested, placing his feet apart and hands behind his back in regimental style. At the mess table he used a knife and fork in a proper manner and cleverly used his drinking basin.
-- Except from the article Jackie The Baboon by the South African Military Veterans Organization Of Australasia (SAMVOA).
As explained in the article, Jackie served in the front line, and was particularly good at detecting enemies.
In April of 1918, Jackie was injured by a piece of shrapnel during a confrontation in Reninghelst, and lost his leg.
Although Jackie served in the front line, I have found no account of Jackie engaging in combat. As for his skill with knifes, as mentioned above and portrayed in the picture, Jackie was able use eating utensils... this does not imply its use as weapon.
Jackie died a day after a fire destroyed the farmhouse on 22 May 1921 and Albert Marr passed away at the age of 84 in Pretoria in August 1973.
-- Jackie The Baboon by SAMVOA.
Monkeys handling weapons
As we can see above, monkeys can pick and use a knives and use some extend.
Pay attention to the way Monkeys instinctively pick knives. They do a Reverse Grip (a.k.a Ice Pick Grip), which is effective to make a strong stab, but not for reach.
Also, see that a knife can be a large weapon for a monkey:
Due to its size, one would expect that the monkey would require both limbs to be effective. Also, we shouldn't expect the monkey to be good at fencing (disregarding the required training) just because this weapon is not appropriate for them.
This all makes Jackie exceptional.
The above suggest that it could be a good idea to provide the monkey with a shorter (and perhaps slightly curved) blade that the monkey can conceal in using the reverse grip. Perhaps something designed to be easily carried in their mouth when they are running (they can run on three or even two legs, but not as effectively).
In fact, you can train them to walk (not run) in two legs or on the hands, or even spin a fire pole.
Therefore, you carry your well-trained monkey... and sneak attack! Monkey jump to the back of the victim and quickly stab the neck before they can shake it off... R.I.P
You can have a monkey probably not betray you by being fair and useful to it. Monkeys understand fairness and have moral principles. Under thar order of ideas, if you treat them as people and you provide for them (shelter, food, attention, etc...) they will have the incentive to care for you and could defend you.
With that said, the monkeys probably will not attack unless they see others attack you. That is because, they usually resource to make noise and throw stuff to scare intruders off. Yet, once in a fight, they can be deadly.
Training them to attack is more complicated, you would have to set up a simulacrum for them to practice (I am picturing some straw stuffed dolls hanging from the ceiling, or stuff like that) and a rewards for their work.
Will they betray you... maybe, probably, perhaps? Will they kill you? If they see you as a threat, most likely they will. Consider this: humans can betray you, why wouldn't monkeys?
Monkey against human troops
Even under the assumption you can reliably command them to attack, a solider with shield, spear and sword can effectively defend itself against a monkey. For your monkeys to be effective, you need to ambush the enemy.
You need the sneak attack. Instruct the monkeys to attack the encampment when the soldiers are sleeping, or have them jump from the trees when they are passing by. If you need to defeat guards, have the monkeys sneak around their field of vision.
Think less of them as monkey soldiers, and more as monkey ninjas.
Although Chimpanzees are not monkey, they probably most useful in battle. Chimps could also be trained for combat!
► Charlie, The Karate Chimp ! (watch video)
We can also see that chimps naturally use sticks and rocks as weapons:
Chimps using sticks as weapons (watch video)
Weapons (watch video)
Apparently, chimps seem to be a very good option for a combat unit, although they probably are more effective with a spear than a knife, given that in nature they often use branches as weapon. Consider creating and designing custom weapons and armor for the chimps.
The following videos where released in 2011 as supposedly real footage from the 20th Century Fox Research Library. The videos "went viral" shortly after.
Chimp with Machete (watch video)
Ape With AK-47 (Gun) (watch video)
Rise of the Planet of the Apes by 20th Century Fox was released later the same year.
Are you still not convinced they are fake? Well, the article Behind the Social Marketing of ‘Rise of the Planet of the Apes’ documents the campaing. The full article is behind an account wall, but you can still read:
To raise the film’s profile online, Mekanism both targeted a group of 50 social media influencers as well as released a series of short “real ape” videos they hoped would go viral. The former campaign began back in June, when Mekanism reached out to selected YouTube stars and movie-bloggers to help them build buzz — especially among millenial-age males.
Can a chimpanzee learn to shoot a gun?
Regarding the videos, LiveScience asked John Mitani, primatologist from University of Michigan, who said:
I wouldn't doubt that you could train a chimp to wield a gun in the manner shown.
When shooting the gun, I'd be hard-pressed to think that the chimp can really understand [the consequences of] what he's doing.
Also Steve Ross, primatologist at Lincoln Park Zoo in Chicago said:
Chimpanzees have been seen to use rudimentary weapons (such as projectiles, clubs and spears), so they have the capability of understanding that a tool can be used to cause harm or do damage.
Whether or not they would understand a gun is more difficult to say.
Source: Planet of the Apes: Can Chimps Really Shoot Guns?