I tried finding that in the Manual>Alliance wars and on several other possible places, probably wasnt looking on the right place. When an alliance can declare a war to an alliance to which it had already declared a war in the past? Is there a period of time during which it cannot declare a war to the same alliance again?
Yes, in the moment the war ends, you can redeclare it.
Its interesting because i had the opposite problem. We lost a war against "re_pulenta" (the pillaged us to death) with 15 points difference, and the whole alliance was fighting. 2 weeks later i had an alliance with only 5 players (the others left), declared war to them (re_pulenta) again, fought completely alone (my allies simply lost 2 battles) and won the war with 832 points difference.
And i got all the war resources