Many answers have been given. They are good answers, considering different perspectives, but I think that most objections can be solved when you observe the current context.
Since we're talking about the impact of shapeshifting soldiers on a modern military, I think we can make a few other rules:
- Anything less crazy/advanced than a human shapeshifting into a T Rex should logically be allowed.
- Anything that is based on scientific principles that the shapeshifting itself relies on should logically also be allowed.
- Shapeshifting into a T Rex becomes useless if the benefits of doing so can be achieved by using cheaper and simpler methods (e.g. comparing it to a tank).
I'll refer back to these rules as justifications for later statements.
There are many things to consider here, I'll try to address all that I can think of.
1. Shock effect.
While the shock effect will indeed work, this will be temporary. The enemy will get used to it, they will prepare their troops, they will arm themselves appropriately (thus lowering panic from being unprepared)...
There may be some residual shock from seeing a T Rex charge at you (a basic fear instinct), but any adequately trained soldier is trained in suppressing their instinctive responses to fear.
2. Open warfare.
I think there is little benefit to be gained here, compared to what armies already have available.
- Destroying buildings (rule 3: you can use a tank or explosives)
- Bullet sponges (arguably not a benefit, it expends super soldiers which may financially bankrupt your military) (rule 3: you can use a tank or something like a riot shield)
- Massive target
- Likely unable to wield any weaponry nor armor (since that would require a soldier carrying around T Rex sized armor)
- Limited to melee range
Due to their melee range and massive size making them an easy target, this is the equivalent of taking a knife and no ballistic armor to a gunfight.
This may actually yield a benefit. One of the major issues with infiltration missions is that you need to pack light. This means that if you are spotted, you don't have much equipment to defend yourself.
Infiltrations take place in places that are heavily defended, but generally are not actively in combat at the time of infiltration. This means that a tactical strike may allow a T Rex to wreak havoc and shapeshift out of sight before the guards have a chance to counterattack.
This is more a question for you: what happens if a soldier shapeshifts into a T Rex when he is in a small room? Do the walls collapse? Does the soldier fail the shapeshift? Does the soldier die because he can't achieve a full form (that is biologically sustainable)?
If the soldier breaks the walls, you could have your soldiers let themselves get captured, only to then rip the inside of the enemy compound to pieces. Even if you don't intend your soldiers to get captured, this seems like a dramatic upgrade from e.g. a cyanide pill: your soldier still dies but has also massively damaged the enemy's compound.
4. Shapeshifting problems.
The biggest issue that the answers have raised is that you'd be naked when you shift back into human form. I don't think this is necessarily the case. I can see two arguments here that make sense.
Firstly, the soldier could wear a custom material that can deal with the transformation. If your soldiers are capable of shifting back and forth without issue, I would expect that your scientists have been able to create a material that can sustain a similar repeated in/decrease in size.
Rule 1: If you can already make living tissue deal with size changes, you should definitely be able to find a non-living material that can handle the same thing.
_Other fictional universes have solved this by making a special material that responds _just like your own living cells__ when a transformation starts. I.e. your body's cells cannot see a difference between themselves and your body suit's cells, thus including them in the shapeshifting process (whatever happens to your body's atoms also happens to the suit's atoms)._
Secondly, your soldier may be able to "store" his (non-shifting) apparel. Your T Rex shapeshift isn't just a matter of increasing/decreasing in size (e.g. like the Incredible Hulk). Your soldier is physically altering the structure of his body. His skin and skeletal structure will vastly change.
Rule 2: If your soldier is already capable of reconfiguring his own molecular makeup, he should be equally capable of reconfiguring his molecular makeup in order to account for his apparel. Since he's already changing his molecular structure, he could morph his body around the suit, thus storing it internally in the T Rex' body. There are a few ways this could be achieved:
- If your soldier is capable of shifting his insides around (molecularly), he should be able to shift the apparel into a little pouch somewhere.
- If your soldier is not capable of shifting his insides around (molecularly), then it's possible for the apparel to stay in place. E.g. if the human soldier is the right leg of the T Rex (and the rest of the T Rex body grows from there), then the apparel might still be inside the T Rex' right leg.
You have a double whammy here.
The first one is relatively simple: Most armies are limited by their logistical capability. Having your soldiers transform into T Rexes is the logistical equivalent of having your soldiers transform into (fuel-free) transport trucks. It pretty much solves all logistical problems (at the cost of being less armored and more visible).
Rule 3: when you really need armored vehicles or don't want to draw attention, simply use whatever you were using before the advent of the T Rex super soldier.
Rule 1: Or you could simply develop an armored suit for your transport T Rexes.
The second one is more insidious.
Remember how I mentioned (in chapter 1) that the enemy will eventually prepare to battle with T Rexes? One answer here correctly pointed out that an M203 grenade would make short work of a T Rex. And that is correct. However, do not that you'd need at least one grenade per T Rex.
That means that your enemy needs to have a massive amount of grenades available. They would essentially need a grenade for every potential T Rex that you bring into battle (and that's not even including missed shots, extra supplies, or lost ammunition).
This will massively affect your enemy's army's logistical capabilities. Grenades are much larger than bullets (obviously), and need to be handled with more care. Both of these will lowers the amount of grenades that their logistical system can provide to a region.
Your enemy will then be less eager to engage you in combat if they don't have an adequate supply of grenades, which is harder to come by than bullets. Secondly, this makes their warehouses more explosive (due to many more grenades), which can be a weakness if you target their ammunition stores.
Logistically, this does make sense.
6. Playing your enemy.
I watched an episode of a Cops-like show yesterday. The cop made a valid point when he was unable to catch a fleeing thief on foot.
Criminals come in all sizes. Slow and armed, fast and unarmed, defensive, offensive, carrying chemicals to disable officers, willing to kill, ... Police officers need to be prepared for all situations. Criminals only need to be really good at one of the situations to win. Cops are at a disadvantage because they have to be a jack-of-all-trades, and therefore are unlikely to outmaster a criminal in a particular field (other than sheer numbers).
The same tactic is true here. The fact that your soldiers are able to shapeshift means that your enemy needs to prepare for that eventuality. But that doesn't mean that your soldiers are required to actually shapeshift.
Even if there are countless drawbacks and only one fringe case where shapeshifting grants a tactical advantage, that means your super soldiers are still capable of assessing whether or not they should actually shapeshift. It's a tool in their arsenal that they can choose to use.
Unless these super soldiers have a weakness (in human form) that non-super soldiers do not have, there is no drawback to having the ability (other than the financial ramifications of turning them into super soldiers, of course)
7. Patrolling and surviving the elements.
Are your T Rexes capable of anything a T Rex is? Because that would make them useful for patrolling large regions.
They could even rely on eating local wildlife rather than having to take provisions if you send them into a large jungle/forest.
Given that they retain their human intelligence, that also means that a super soldiers is able to shapeshift based on what suits them best:
- Cold weather? Stay in human form, you're warm-blooded.
- Hungry? Become a T Rex and hunt wildlife.
- Enemy soldiers trying to stay hidden? Have your squad go half human and half T Rex; the enemy will need to avoid two completely different types of enemy in order to stay undetected.
8. No cheesing.
Not a real chapter, but I wanted to list things that I omitted not because they wouldn't work, but because they are too cheesy for narrative purposes:
- Utilizing the mass difference between a human and a T Rex to cheat physics. E.g. imagine balistically launching a human (low-energy) but having him land as a T Rex (high-energy).
- Similarly, no power generation or other ulterior benefits from the change in size/mass.
- Other than the shapeshifting itself, no superpowers that T Rexes and humans do not have. Also, humans are limited to human capabilities, T Rexes are limited to T Rex capabilities (no speaking T Rex, no human that can bash through a building)
- No mysterious animalistic skill whereby a T Rex can expertly command other animals. This would also violate rule 3, there's likely a much cheaper way to coax animals (e.g. build a fake but convincing T Rex model).
In essence, there are benefits to having T Rex shapeshifters. However, they are not likely to be found in open combat, but more in the auxiliary aspects of a modern day army.