US factory upgrade timing issue, after being provoked.

My understanding is there are three ways the US enters the war.  It would be interesting if there are more.

1.  US gets attacked by any one of the axis powers.
2.  London falls.
3.  Round 4 they are automatically at war.

Not sure I understand the other part of your question (not that I would probably know anyway).

It’s an immediate upgrade as the rule says. You upgrade during Japan’s turn if Japan declares war.

Round 4 they may declare war on any/all axis at the beginning of their collect income phase. They are not required, but definitely should do so unless looking for the fabled isolationist victory.

To get this straight, though. If directly attacked, the US is at war on the attacking powers turn. The industrial complexes get upgraded immediately and can be used on the US next turn

If an ally is attacked in a way that allows the US to come into the war. The US must still declare war on her turn at the beginning of the conduct combat phase.
This is where it becomes ambiguous. The US did not purchase the industrial complex to be deployed during the Mobilization phase as would normally be the case.  She didn’t have the major complex at the beginning of her turn (though she did possess the minor complex), but there is no restriction saying that you must purchase only the number of units you are able to place.

So the question is does the US meet the requirements to utilize the major complex, which  are

1 Must not have captured or purchased the IC this turn
2 You can use an industrial complex that you controlled at the beginning of your turn

The US meets 1 clearly, but strictly speaking does not meet 2 (did not contrll the major complex at the beginning of the turn so can’t use it)

From Larry’s Site  Rule Clarifications:

Mobilizing New Units

When upgrading a minor industrial complex to a major one, only three units may be mobilized by the complex in the same turn (not counting the upgrade itself).

The only time this matters, is on US3.

On US3 during the COLLECT INCOME PHASE - The USA can declare war, and collects it’s National Objectives.

That is the only time this concern about major/minor factories will matter.  And said factories are upgraded AFTER unit placement - thus limiting them to 3 placement slots on US1 - US2 - US3.

@TITANS:

so by saying “at war”, it means being involved in fighting, even if you haven’t had the chance to make your declaration of war yet.

Perhaps this quote from the Official Rules Clarifications will clear this up:

If another power declares war on your power, your power is at war immediately, but only with the power that declared war on it. You must wait until your turn to make any declarations of war on other powers that were enabled by that declaration. For example, if Germany declares war on the United States, the United States is immediately at war with Germany, but it must wait until its turn to declare war on Italy. Of course, the United States may also declare war on Germany on its turn, but this is technically unnecessary, as the two powers are already at war.

@SovietOnion:

So the question is does the US meet the requirements to utilize the major complex, which  are

1 Must not have captured or purchased the IC this turn
2 You can use an industrial complex that you controlled at the beginning of your turn

The US meets 1 clearly, but strictly speaking does not meet 2 (did not contrll the major complex at the beginning of the turn so can’t use it)

Welcome, SovietOnion!

The use of the word “immediately” in the rule regarding US IC automatic upgrades is intended to imply that the increased production capacity may be used in the US’ next Mobilize New Units phase after it enters a state of war.  This overrides the normal upgrade production capacity rules, as the upgrade is not put into effect during the Mobilize New Units phase.