They're not unusual for societies that have an enemy who wants to kill them on the same piece of land and no natural frontiers in the middle, no. Ie, not the UK. The greater the distance between an invader and a country they want to invade, the less likely they are to pull it off absent a huge difference in technology available like the Spanish and the Incas etc.
And no, I don't think there's the remotest likelihood of Russia and China attempting to invade the UK. You may have noticed that Russia's most recent land invasion hasn't quite gone to plan. When they try and target us, and you must know that's what Russia have been doing, they use more sophisticated methods. The sort that a load of unwilling, possibly hungover adolescents aren't going to be much use countering. I'd be more worried about them trying to target the infrastructure, which can be done without any boots getting anywhere near us.
It's actually really important to be clear about this, because our Article 5 obligations are important and we do need to firm up our defence. Imbecile pronouncements about compulsory national service from Sunak aren't the answer to this, they're an attempt to distract from it. Because then we might start asking questions about what the fuck his party have been doing for the last decade and a half.