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In the Iraq War and the war in Afghanistan, two of the biggest vulnerabilities of occupying U.S. and allied forces that the local resistance in Iraq and Taliban in Afghanistan used were to shoot exposed gunners with snipers, and to use improvised explosive devices (IEDs), as land mines (sometimes controlled visually by cell phones), to exploit the flat bottoms of Humvees, tanks, and other armored vehicles.

Humvees weren't designed to be on the front lines, but ended up there in wars that had no front lines.

Armored vehicles, including tanks and infantry fighting vehicles, were designed to have side armor protecting them from direct fire in a two dimensional battlefield, and had the heaviest armor on the front where their opponents were supposed to be.

In the moment, the U.S. bought mine-resistant ambush protected (MRAP) vehicles to respond to those threats. These featured curved rear surfaces to deflect blast energy from mines and make IEDs less effective, and either protected or remote controlled gunner positions to protect gunners from ambushes.

But neither the M1 Abrams tank, nor the M2 Bradley Infantry fighting vehicle, which are the mainstay of U.S. Army armored forces, were redesigned to provide mine and IED resistance, although some upgrades to protect gunners from ambushes and sniper fire were adopted.

The Joint Light Tactical Vehicle (JLTV) that has replaced the Humvee has the features of the MRAP, but more thoughtfully put together without the time pressure of a war being conducted.

The Ukraine war has cast attention on another of the problems with almost all armored vehicles in protection. They almost all have lighter armor on top, because the threats that their armor was designed to stop were expected to come from opponents, like other tanks, in front of them, or at most, on the sides in a two-dimensional plane. So, almost all existing armored vehicles are especially vulnerable to anti-tank missiles and artillery rounds that strike from above, and to bombs dropped from drones and loitering munitions from above, relative to strikes from the front where their armor is strongest. Tens of thousands of armored vehicles in Ukraine on both sides of the conflict, modern and dated alike, have fallen to top side attacks.

A new generation of front line military vehicles and systems need to be defended not only from the 360 degrees surrounding the vehicle from all sides in a two dimensional plane, but also from threat below, like land mines and IEDs, from above, and from all angles from low to high in between, including fire from the side from ditches, and fire from above from tall buildings or mountains.

The Limits Of Armor

Of course that's not the only design issue facing armor in an era of new threats.

Offense has surpassed passive defenses for the foreseeable future. The maximum feasible amount of armor protection comes with immense weight, makes it hard to deploy to the conflict quickly, slows the vehicle down, makes it vulnerable to mud or pits and trenches, and makes it less fuel efficient with a longer and vulnerable logistics supply chain. But even maximal armor can be overcome by much smaller, lighter anti-armor missiles. And, lesser strikes to tracks on the vehicle, for example, can produce soft kills that force troops to abandon it.

While armor is invincible against small arms fire and shrapnel, and provides some meaningful protection against lighter anti-tank weapons like 20mm to 50mm canons and RPGs, no amount of armor is going to prevent a direct hit from an anti-armor tank round, an artillery shell, an anti-tank missile, and a guided bomb dropped by fighter aircraft or a bomber. For those threats, the only defenses are to be out of range, to be undetected, or to have active defenses.

When it comes to deployments, airlift and transportation of armored vehicles by rail and road bridges built to civilian standards in most places where the U.S. might go to war is greatly impeded for vehicles with more than 38 tons or so, while vehicles of 19 tons or less are much more easily deployed by air (including a C-130) and across civilian road and rail bridges.

The Problems With Tracked Vehicles

The same considerations that disfavor heavy armor also disfavor tracked vehicles. Tracked vehicles top out at about 45 miles per hour and that simply isn't fast by any measure. At that speed, even trying to get out of range of incoming opposition forces is basically futile. And, any military unit with even a single tracked vehicle is limited to the speed of its slowest vehicle. The last sixty years of experience have shown that the off road capabilities of tracked vehicles are utilized much less than planners anticipated (and again, a whole unit can't travel off road unless every vehicle in the unit has that capability), and the comparative disadvantage of wheeled vehicles designed to be used off road against tracked vehicles has greatly declined. Also, slow, heavy, tracked vehicles are hard to hide and not being seen by opponents who can destroy you until it is too late is critical in modern ground warfare. A drone can easily see where a tracked vehicle has been and trace that to its target. Tracked vehicles use at least twice as much fuel per mile as comparable wheeled vehicles.

Tracker artillery vehicles and missile launchers are ill suited to modern "shoot and scoot" tactics.

Active Defenses

Active defenses, in contrast, have made immense leaps and bounds. Electronic warfare devices can disrupt the controls of remote controlled or GPS guided weapons. Automated active response systems can shoot and defeat incoming drones, missiles, and shells before they hit an armored vehicle, leaving its armor to merely deflect shrapnel. Long range anti-air and anti-tank missiles can defeat incoming ground attack aircraft, attack helicopters, drones, and enemy armored vehicles before they are within the range of direct fire weapons like the main gun of a tank, cannons, rocket propelled grenades, recoilless rifles (a.k.a. bazookas), or other enemy weapon ranges.

Vulnerable Logistics Supply Lines

Logistics also takes on a new relevance in an era of war without true front lines. No military unit can get far without supplies of fuel, ammunition, food, and water. The heavier and less fuel efficient military vehicles are, the more fuel they need. The more "dumb" their weapons are, the more ammunition they need. But in a world without front lines, logistics supply trucks are vulnerable because they are softer targets than armored vehicles. Increasingly, however, logistics vehicles need to be as protected against hostile fire as the vehicles which were traditionally the tip of the spear. So, we need to minimize the size of the logistics supply line and to make the vehicles that are a part of it more survivable, to at least the point of armored personnel carriers.