The fighting in Syria/Iraq and in Yemen is tossing military theory into the trash bin.
What do I mean?
We're looking at mechanized forces being beaten by light infantry on an almost daily basis. If you take a step back and look at the "order of battle" for all sides involved, then we should see Iraq curb stomping ISIS in their country, Syria is a bit more muddled, but in Yemen we should see the Saudis/GCC having wrapped up military operations by now.
Instead we see at best military setbacks and at worst a stalemate with the 'insurgent' forces (I use this term for want of a better one).
How can this be? If we say that its because the mech forces have poor troops then we must also acknowledge that they're facing villagers with a few professionals mixed in. At best the troop issue is even. If we look at tech then Iraq/Syria/Saudi-GCC certainly have a huge edge. Add to all this the available airpower that they all have then we must start asking serious questions about what's going on here.
NOTE: I didn't delve into tactics because while I believe it to also be an equal factor, I'm sure many of you will consider it the cause of their failure. I don't think so and for this discussion lets follow my lead and consider both sides to be sloppy in their TTPs.
Note 1: Is it possible that we've seen the demise of not only the tank but also armored vehicles on the battlefield? Again, I don't think so. But I do believe that we might be reaching a point where it takes extremely professional Soldiers/Marines to successfully operate them on hybrid battlefields.