Improvement in technology and access to cheap drones. This counters a huge advantage that well-funded militaries used to have, which is precision munitions. Such munition is expensive due to autonomous guiding which has to ingest, process and act on outside information, which used to be extremely expensive to do. Drones allow offloading of the processing and action to a human controller who is cheap, because he/she does not get destroyed with the weapon and can be employed many times.
Both Ukraine and Iran have allies who provide them with satellite-based intelligence. That means they can have almost real-time view of the battlefield, can target their drones accurately and do considerable damage with quick damage assessment and possible re-strike if needed. Satellite intel is generally available only to major powers or superpowers, but with willing allies, even a small-budget military can get access to it and use it to deadly effect.
Both of these advancements dispoportionately favour the defender compared to the aggressor, as evidenced by the two wars in Iran and Ukraine. The defender is capable of targeting enemy bases and larger groupings of units, whereas the attacker is hard-pressed to find distributed defenders. I think that successfully conducting a war of conquest is nearly impossible these days against a determined nation.
I think that there are two aspects of this:
Both of these advancements dispoportionately favour the defender compared to the aggressor, as evidenced by the two wars in Iran and Ukraine. The defender is capable of targeting enemy bases and larger groupings of units, whereas the attacker is hard-pressed to find distributed defenders. I think that successfully conducting a war of conquest is nearly impossible these days against a determined nation.