Everyone is correct!
Pretty much since humans have been organizing themselves for maximum efficiency in the most wasteful, unproductive, and destructive activity we can think of–war–the battlefield has been dominated by the idea of combined arms. The concept has evolved and militaries have refined it a lot over time, but the essential idea remains. You need a multi-spectrum approach to warfare if you hope to succeed. Even something as simple as you guys stand their and beat on them while you guys go over there and throw rocks is a primitive yet effective form of combined arms.
Light, mobile forces are essential for some things, and not so useful for others. Ditto heavy forces. Hammers and anvils, holders and flankers, line of battle and skirmishers, there’s a place in the House of Mars for everyone!
In all seriousness, though, the US has wrestled with the problem of force balance for a long time. In the very specific circumstances of Central Europe during the Cold War, the focus was rightly on heavy forces. Light forces, except for screening, recon, and rear area security, seemed unsuitable when facing a potential armor-heavy Soviet juggernaut.
The end of the Cold War left us with a force structure built for a war that never happened. When we needed to deploy forces to the Gulf in Desert Shield/Storm, it took a long time and a lot of logistical work to shift those heavy forces to the desert. In very few scenarios though would we have that time, as well as a friendly staging area like Saudi Arabia.
Until 9/11, there didn’t seem to be much consistency in our planning, or any clear direction, but the deployment of forces to Afghanistan reignited the calls for lighter, more mobile, easily deployable forces. After all, fighting Al Qaeda or the Taliban hardly required the 1st Armored Division, but it did require a lot of grunts with rifles, mortars, all-terrain vehicles, and helos. But then in 2003 we invaded Iraq, where it seemed heavy forces might be useful again. After we toppled Saddam, though, once again it was a sort of low-intensity conflict with a lot of fighting in built-up areas. More infantry and light force focused stuff.
So in the early 2000s the debates kept going, with a lot of development of new gear focused on things like mine-proofing, rapidly deployable forces, lighter formations that could be sent hither and yon quickly. In the back of many minds though was the persistent thought that if we did have to fight someone with a heavy force structure, we couldn’t let our own atrophy too much. A lot of factors make focusing on lighter forces easier though. More contractors can participate than when you are talking about tanks, or self-propelled howitzers. Prototyping and procurement in general is often easier. The stuff is usually cheaper, though not always. And most of all, until the Ukraine invasion really no one actually expected a large-scale land war using heavy armor in quantity.
So, I imagine the E ring conversations about now are pretty interesting concerning force development. On the one hand, drones, impromptu light strike teams, special forces, and artillery seem to be very effective against Russian tanks. OTOH, tanks on both sides also seem to be very useful for a host of missions, and it is quite possible that the success of light forces is made possible by the threat of Ukraine’s own heavy armor formations; this stuff isn’t happening in a vacuum.
