The way we wage war in the 21st century is likely to be upended by the fighting in the Ukraine War. The Russian invasion of Ukraine may cause fundamental changes in how war is conducted. What can the Western militaries learn from this war?

The success of the Ukrainian forces against Russia armor is extraordinary. The Ukrainians destroyed thousands of tanks, armored vehicles, and heavy trucks. The hand-held anti-armor weapons provided by NATO countries are to blame.

It is indicative of a tactical approach by the Ukrainians that combines the intelligence provided by the West with the new systems like the Switchblade drones, as well as small, light special forces teams.

Each armored vehicle destroyed means more dead Russians. The number of Russian soldiers killed in action is staggering. The U.S. lost 7,000 in Iraq and Afghanistan in twenty years. The armor stocks will be hard to replace in the short term. Each missile costs less than a hundred thousand dollars, but each Russian tank costs more than $10 million. General Sherman said in the American Civil War that war is expensive.

Is it time to say goodbye to the tank on the battlefield? Will they become obsolete by new technologies and tactics? Reducing tank inventories and using the resources to move toward new systems is something the U.S. Marine Corps is already doing. Tanks can still be effective, but must be used in a way that protects them from such mechanisms.

The concept of close air support is in danger. There are concerns about the vulnerability of helicopters. Russian attack helicopters are being destroyed by a hundred thousand dollars. This tactic allowed the Afghanistan mujahideen to defeat the Soviet Union. The economics of this, especially for a weak economy like Russia, are daunting.

New swarm drone systems will come into full force after that. As artificial intelligence becomes a wartime reality, it will be possible to control large numbers of drones and operate them in a way that will attack less maneuverable platforms. We are at the leading edge of achieving this capability, and doing so would be bad for expensive manned aircraft, especially those that operate near to the ground.

The lesson isn't to completely walk away from manned aircraft providing close support on the battlefield. The Ukrainian war is a warning that we should be spending more on research and development that improves the effectiveness of our air systems, both in ground attack and in anti-air capacities, and experiment with such capabilities aggressively to be able to provide close air.

Ukraine is our past and our future.

The ability of Western intelligence systems to track Russian formations and provide real-time targeting directly to the Ukrainians has been a key factor in the Ukrainian battlefield. This has led to high levels of Russians being killed in action and operations killing Russian general officers. There are many reports indicating a lack of control on the battlefield and operations being directed from Moscow.

When I was NATO's Supreme Allied Commander, I didn't think I'd take control of 150,000 troops in the field. That is what appears to be happening in Ukraine. The lesson is that by providing real-time, highly precise targeting to forces in the field, a belligerent can help undermine one of the true centers-of-gravity in combat: a coherent command and control system anchored by capable senior leaders.

We should learn from the Russian example of war crimes. We need to be aware that our opponents are going to use horrible tactics that are war crimes: destruction of civilian infrastructure, including internet and cyber systems, false flag operations, weaponizing civilian populations, and taxing the logistics.

We have seen the Russians go to this list of dirty tricks in Afghanistan, Chechnya, and Syria. We need to do a better job of preparing for these new realities on the battlefield. We need to reverse engineer and blunt these techniques in our training and equipment choices. Training our troops to operate in chemical and biological environments more effectively, providing more civil support to local populations, sharpening our ability to collect evidence to undermine fake videos and propaganda, and honing our responses to battlefield cyber attacks include offensive options are some of the things this means.

Special forces, drones, and cyber will be important in the 21st century battlefield. We need to rethink our way of war because legacy systems like tanks and destroyers will retain utility. There is a lot to learn from the battlefields of Ukraine.

We can be reached at letters@time.com.