To be honest I’m not that convinced the size of the force is an issue; what matters more will be based on how rapidly we can build on what we’ve learnt from Russia’s Special Military Operation:
1. Rapid reaction tactical forces to hit a land invasion where it hurts - emphasis on “rapid”. But these groups need to be hard as nails, and highly professional. If the invaded region has to rely on civilian call-up, then the invader has the upper hand.
2. New ways to interrupt the supply lines for an invasion force - a well kitted out drone with EMP would frag the enemy comms.
3. Drones and UAVs. Why put boots on the ground when a cheap bit of expendable kit can put a laser marker on a target without even being spotted?
4. Countermeasures, and retaliatory capability, to hit all forms of artillery.
5. Air support. Lots of it.
6. Propaganda - the worst thing about this war is Russia being able to wall off their own people, to the extent that they can pump out 24/7 propaganda on the internet, national television and radio without people having access to other sources of information.
7. Decentralised energy and telecoms supply chains. It’s far more difficult to knock out the infrastructure if it’s higly mobile and highly distributed.
8. A willingness to launch counterstrikes that scare the pants out of the invading nation. There’s no point just grinding down their troops if the message isn’t getting to the genpop. Plop a bunker busting non-nuclear rocket onto the roof of Putin’s country retreat, ideally via a hypersonic that Russia doesn’t even think we have.
9. Don’t broadcast the locations and tactics to the enemy. I seriously can’t believe how many times that’s screwed things up for both sides - the best one being when the Russian tiktok loonies posted a brag about their awesome new launcher, only for the Ukes to see it, geolocate it, and take it out in less than 24 hours.
An awful lot of this is down to savvy use of comms.
It took over 2 months for Russia to take control over mobile comms even in the areas they do hold, and even then they can’t block all social media. So it’s very difficult to control the narrative without hurting their own communication lines.
If the Ukes had had the ability to take that very long convoy out comprehensively before it was half a day into Ukraine, while Russians still had the internet, Putin’s censors would’ve had a right tough job denying it.
Threatening to nuke Russia if they step foot onto NATO territory works just as well. Same goes for China on Taiwan and North Korea on South Korea.