With the new expansion, the tactic value now let's you reroll dice. It says once per "Attack". The words attack and round are confusing me. So a battle that lasts 2 space battles and 2 ground battles, how many times am I allowed to reroll dice? Sorry if this is a repeat question.

from my understanding, it's per attack. So you get your full re-roll each space and ground phase. Unless someone wants to correct me? By no means an expert.

That's what I thought too as the leaders should be useful.

Maybe with the definition from the RRG it's clearer:

"Combat Round: Players resolve a combat round in which each unit performs one attack... "
"I Space Battle: The current player resolves one attack with all of his ships. Then his opponent resolves one attack with all of his ships."
"II Ground Battle: The current player resolves one attack with all of his ground units. Then his opponent resolves one attack with all of his ground units."
...
"Next Round: If both factions still have units in the same theater (space or ground), they resolve another combat round , starting with the “Space Battle” step. Otherwise, the combat ends."

So you get to reroll once per arena, per combat round. For example: combat round one you reroll once for the space battle and then once for the ground battle. Then when you attack again on round 2 of combat you get to reroll again for both arenas, etc.