Did you notice there are some ways of blocking enemy reinforcements?

I'm talking about Total War: Rome 2 specifically ( I don't know if the same techniques would work in Total War: Attila )

Even within reinforcement range is possible, believe it or not.

- One way is to use a spy: let's imagine there is an enemy army inside a city ( In case you want to take the city immediately without waiting for any turn ). And another army outside the city but within reinforcement range. What you have to do is bringing your spy and sabotaging ( I mean the action of sabotaging enemy supplies ) the enemy army that is outside the city. And if the sabotaging is succesful, just attack with your army the city. And trust me guys: the enemy army that is outside the city won't join the battle.

- Another way is using two armies in the same case as above ( there is an enemy army inside the city and another one very close to the city ): If you want to avoid facing both enemy armies at the same time, you just need to to lock the army that is inside the city: what you have to do is using one of your armies to attack the city but don't start battle yet. Leave it in siege mode. Then use your other army to attack the army that is outside the city. This way the army that is inside the city won't join the battle and not even the garrison army.

- Another scenario is in the open: when there are two enemy armies very close to each other and you have only one army but you don't dare facing both armies at the same time in battle in the open. Ok. In this case you need a spy. Just sabotage the supplies of one the armies ( the trick works only if the sabotaging action succeeds ). And with your army attack the army that wasn't sabotaged by your spy. This way you'll face only one army at a time.

- And if you are a barbarian faction, by choosing night battle you can also block reinforcements as in the scenarios above. But the negative side of choosing night battle is that in case you have 2 armies close to each other, your other army won't come to help you.

Following let me share two examples of enemy reinforcements being blocked.
- Example A: watch it from 57:02 to 1:16:12:
https://www.youtube.com/watch?v=b0o-WMC9-CA

-Example B ( From 1:33:21 to 1:33:58 )
https://www.youtube.com/watch?v=8yVSw2VnRVk