Warfare is a constant tug of war between offense and defense. We are just a little bit past the pinnacle of offense in that ongoing battle. A 70 ton Abrams tank (which weighs more than 140,000 pounds), which owes its immense weight primarily to its armor, can be destroyed with a well placed hit from a 70 pound Hellfire missile. Torpedos or cruise missiles, which weight about 1 to 2 tons, can destroy nearly 10,000 ton destroyers, notwithstanding their armor plating. A 500 pound roadside bomb can flip a 25 ton armored vehicle.
A new class of point defense technologies, however, are on the verge of upsetting that balance. We don't yet have any technology that can reliably shoot down intercontinental ballistic missiles directed at any place in the continental U.S., as the national missile defense that Republicans since Ronald Reagan have been pushing demands. But, several technologies on the verge of introduction can defeat incoming missiles, mortar rounds and artillery shells, or at least, identify the source of incoming projectiles. The technologies that do this are varied, although all rely on some sort of computer technology to target incoming projectiles faster than any human could.
* One of the earliest of these technologies to receive prominence was the Patriot Missile system, which destroyed several Scud missiles in the Gulf War.
* The Canadian "Ferret" technology is a projectile locator without any active defense. It is intended primarily has an anti-sniper tool that can locate instantly the source location of even a single incoming bullet.
* The THEL system combines targetting technology with a laser that destroys artillery shells, mortar shells and missiles as the come in, apparently by triggering the explosives that they carry.
* Another laser system, designed for use on commercial aircraft, renders useless incoming anti-aircraft missiles by frying their guidance systems. Apparently, many of our high end fighter jets can do a similar trick using their active radar systems (an actively electronically scanned array)in a directed way against incoming missiles.
* The Navy's Phalanx Close in Weapons System (CIWS) shoots grenande sized rounds with computer controlled radar at incoming cruise missiles. The system has also been adapted to actively defend against mortors and artillery shells directed at the green zone in Baghdad.
* There is talk of developing similar systems based on a high rate of fire "metal storm" system, or an electromagentic inert round railgun system (such as the one contemplated for the next generation destroyer, the DD(X)).
* Yet another approach would have a computer locate incoming projectiles and have chunks of armor shoot off a vehicle for a short distance towards the projectile so that the worst of the impact explosion would take place further from the armored vehicle.
The great virtue of all of these systems is that they make it possible for a vehicle or ship or military base to protect itself from the kinds of weapons that a rogue nation or insurgent group might acquire without piling on immense quantities of armor. Conceivably, a five ton active defense system might better protect the crew of a lightly armored vehicle better than 30 tons of passive armor on a main battle tank. And, such a system might make it possible to make a "secure" military base or urban safe haven truly secure.
On the other hand, if these system don't work effectively, we could see a very different trend. No one has yet tested the Phalanx CIWS in battle against multiple cruise missiles. Suppose it doesn't work. Then, any nation able to afford half a dozen $500,000 cruise missiles would be able to destroy a $1,000,000,000+ American warship. The mere threat of such an attack, made credible by a handful of real world combat examples, would keep the entire United States surface fleet hundreds of miles away from any nation with that relatively inexpensive capability. Military analysts have stated that it takes less technological savy to build a cruise missile than it does to build a jet fighter, so this is a real concern.
Likewise, a nation that could afford several dozen Hellfire missile knockoffs at $100,000 each could decimate an assault by American main battle tanks before the tanks even got within range of the enemy firing upon them.
And, the mere possibility that ground forces might have mobile anti-aircraft missiles (also in the $100,000 range) has been enough to radically alter the way the United States has chosen to use its air power in both Kosovo and Iraq.
Thus, in short, the continued existence of the surface fleet, low alititude military aircraft manuvers and the continued use of armored vehicles on the battlefield hinges to a great extent on the ability of engineers in the defense industry to develop reliable point defense systems. Either way, military vehicles are likely to get lighter, either to secure greater stealth and speed if active defenses can't be developed, or because active defenses are lighter than passive ones, if they do work. Passive heavy armor will remain useful only against infantry that lack access to missiles and heavy explosives -- keeping only small arms fire at bay.