Tanks and guns are very hard to use. A WW2-era tank requires a 5-man crew (the later models, at least), and the commander needs years of training to be able to accurately assess the situation and tell the rest of the crew, most of whom are effectively blind, on what to do. Apes can't do that.
For example, let's say that you have 5 apes that miraculously find their way into a late WW2 tank, and for the sake of argument, they find themselves sitting in the right places. First of all, only 2 of them can see: the commander and the gunner. The rest are blind.
In order to move, the commander needs to tell the engine guy to turn in a direction and move for a certain distance. Somehow, they manage to do that. Ok, the tank can move.
However, tanks are useless without guns. To accurately fire a gun, you need coordination between 3 people: the commander, the gunner, and the ammo loader. This needs to happen for a shell to hit a target:
- Commander spots a target, and talks to the radio (which, btw, the apes automagically operates): "gunner move to 3 o'clock, estimate 500 yards" (not his exact words, but close enough, I'm too lazy to look up how they spoke)
- Gunner turns his sights to 3 o'clock, and sees the tank. Assuming the ammo is automagically loaded, he fires a shot
- It is impossible to hit a thing on the first try, since the initial range estimate is probably off. The commander sees that the shell overshot, and yells into the radio again: "revise estimate to 300 yards" (again, not exact, but you get the idea)
- Remember, at this point, you need a commander that can manage a crew, accurately estimate distance, and revise that estimate when it's off. Good luck trying to train an ape to do that in 6 months.
- At this point, you need to reload the gun. For this to happen, the ammo loader (yes, modern tanks have auto-loaders, but your apes won't know how to use them) needs to open the gun, take out the hot shell, and then shove in a new shell. Magic happens, and a new shell is loaded.
- Gunner hears the new range estimate, and is smart enough to know, with very limited training, how much he needs to adjust to hit the target, and then fires a second shell, which has a half-decent chance of hitting (if everything goes according to plan)
Do you see how complicated this is? This requires a highly-trained crew that knows how to work together, and operate all the equipment in a tank. A tank is not a car with some armour and a gun; it needs trained personnel and experience to even move.
Back to the forest. Fighting in a forest in hard. Especially when you can burn it down. Oops. Okay, what happens if we don't burn the forests down?
Forest warfare requires deep knowledge of how terrain affects combat and ambush setting/countering, skills which the most intelligent of humans have a hard time grasping. Yes, apes in trees make good ambushes, but sniper rifles can only do so much. Assuming that the apes somehow manages to operate those rifles with the precision required, which again, requires years of training, they still can't inflict much damage.
Also, you know what makes better ambushes? Explosives. Guess who knows how to drop landmines from the skies? Not the apes.
There is really no way for your apes to win. Sorry.