Humans have finally developed a shield completely made out of handwavium. Hurrah! This shield is unbreakable except to anti-tank rounds, RPGS, and other heavy weapons. Assuming the army could afford giving half of their troops handwavium shields, how would this change how modern (2017) infantry moves and takes cover? Assume the shield weighs about 40 pounds and have the same dimensions as a tower shield.
I think it would not change much in modern warfare, as the current trend is less based on infantry combat. They would improve the existent uses of shields in military and police activity, but not change the tactics, as most tactics involve long distance, high-tech (drones), or explosive power. So it would just be used in the same scenarios shields are already used.
The material on the other hand could be used for many different things. Even if you can't forge precise forms you can enhance vehicle plating, body armour in smaller pieces to stop bullets.
But except for guerilla fights tower shields will not have an impact. And there it is only changing effectiveness and not tactics. (Modern shields are already almost impenetrable, after all)
The weapons you describe as being effective against the shields all have a large energetic yield. Then an obvious trend would be to develop weapons that deliver the same kind of impact but are smaller, cheaper and less messy on the battlefield.
Super-sniper rifles might be the order of the day.
A 40 lb shield on top of the average 45 lb pack the infantry is already carrying around would be too much of a burden for most soldiers. Since its made out of handwavium, you could make it a lot lighter.
In addition, the bulk of a tower shield is way too much; and for most soldiers bunkered down in the line of fire they would be on the ground firing back with their profile a lot smaller. Only if they were to do a charge assault would a large shield make it worthwhile.
The shape of a tower shield would not fit the requirements of a trooper lying down. They would need something that works with their weapon:
Again, since its handwavium, you might as well go more high-tech and work on a portable shield that expands when required.
The military doesn't supply everyone with the new hotness shield because some infantry will be slowed down by it. In WW2 soldier packs were supposedly already 90lbs. Do you seriously think that an infantryman wants to carry another 40lbs of cover with them at all times? That's 130lbs.. Weight is heavy.
Right now the units who do use shields in general are swat and riot people. Swat and riot people will get safer, however even they won't all switch to these 40lbs shields because it would make things more dangerous for the people their fighting. Imagine how hard it will be to not hurt rioting civilians if you have to move a 40lbs shield in a shield wall? SWAT doesn't even always use shields anyway. At the end of the day this will become a nice tool that the military has, but it won't be made standard.
Now if you put this in a vest on the other hand it would be used, but the tactics used with it would remain the same as tactics already used with heavy vests today.
Shield-equipped troops, so safe against rifle bullets, meet the device called a "land mine".
Shield-equipped troops, glad you're protected against shrapnel from exploding artillery rounds. Let us introduce you to this thing called "blast wave". Or better yet, this thing called "airburst".
Shield-equipped troops, glad you're protected from people shooting you from the front. Allow us to introduce the tactic called "flanking".
Shield-equipped troops, so glad your shield can stop a bullet. How's it handle a tank driving over you?
Shield-equipped troops, thank you kindly for carrying a big-ass target making it easier to see you to drop napalm on you.
If such shields were adopted, the result would simply be wider usage of mortars and automatic grenade launchers. Mortar bombs can explode behind or above a warfighter as well as before him, thus negating the shield. Grenades just plain destroy the shield and kill its user.
A human being is weak and fleshy, and is limited to converting 3000 Calories into energy per day.
A motorcycle engine can convert 6000 Calories into energy per hour.
A high speed tankette is probably what you're going to end up with. Not Roman Captain America.