The game changer will be swarm capable drones.

A few smart drones can be used to guide a swarm of cheaper drones on target. Additional sensor drones can feed back info to improve this.

Currently, defensive systems can cope with a drone attack. However, if you have 20 coming in from all directions, in perfect coordination, they will be overwhelmed. You don’t even need all of them to be armed, just a couple, protected by the rest.

Current drone usage is akin to the first tanks in WWI. The WWII equivalent will be terrifying.