multi national forces can't attack together (scramble induced defence is still an attack)

[lenzls]

Where in the rulebook does it say that allied units in a SZ don't participate in an attack on that SZ (triggered by a scramble)? Situation: USA holds SZ112, Britain attacks W Germany. Scramble. USA ships shouldn't participate in sea combat. However, I can't find that in the rulebook.

rulebook, Pacific 1940.2, page 20
Multinational Forces
Units on the same side can share a territory
or sea zone, constituting a multinational
force. Such forces can defend together,
but they can't attack together. (This doesn't
mean powers can share income: only the
power that controls a territory collects
income for that territory.)
A multinational force can't attack the same
space together, because each power moves
and attacks with its own units only on its own
turn. Any units in a sea zone in which a battle
occurs that belong to an ally of the attacker
(other than cargo on an attacker's ship) can't
participate in the battle in any way. Such
units can't be taken as losses in the sea
combat and have no effect on defending
submarines.

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