[Taqwus]

Urrrgh. This potentially gets far more complicated than it seems.

* Flying armies.
* Amphibious armies.
* Armies involving a mix of movement types.
* Attacks from multiple provinces including from stealthy forces already on site, possibly including multiple attacking factions.
* Surprise attacks.
* The improbability of a strat-move 1 army being able to commit entirely to defending one bridge while still being able to respond in force on every other border, as well.


Not to mention that if bridges become strategic elements, you really should have
* options to build bridges
* options to destroy them