Britain and France, notably, hadn't really worked out how to use them effectively and so were left fighting the last war, horribly disadvantaged until they worked out how to catch up (though air power being developed at the same time also had an influence).
So now we have drones, and all those incredibly expensive fighter planes are useless. Aircraft carriers will be replaced with cheap drone swarm carriers, so naval strategy will need to change. Anti-tank drone swarms will make conventional armour more or less ineffective. And so on.
The good news is that there will be anti-drone swarms too. And so we'll end up fighting battles mostly by destroying drones instead of people. Once you've knocked out all the enemy drones, they'll surrender because humans can't fight drones.
Interesting times.