When attacking from a region into another region, do supporting units from territories connected to the region the attackers are coming from count toward their total strength? Or is it only supporting units from regions connected to the defender's region?
The following is taken from page 12 of the rules (emphasis mine):
Support can only be given to the area in which the battle takes place, and only if you have a Support order adjacent to the embattled area. Since the March order is played out to completion before the battle starts, the only embattled area would be the area that the attacker moved into (i.e., the defender's area). The concepts of attacker and defender don't even apply until after you've moved into this area.
This is explicitly clarified in page 18 of the second edition rules: