This is a different case. The unit(s) may die due to FTC, not due to retreat priorities combined with inability to retreat. The rules say you cannot retreat units to places they cannot move, but they can move there because FTC is assessed at the end of the step. FTC rules allow units to violate FTC as long as they adhere at the end of the step, FREX, CW air missions over France, RR moves by minors' units though other non-cooperating minors' territory, air transport, etc.
The FAQ describes one and one only situation where units cannot go somewhere due to FTC and that is the land movement step (as proven by FAQ Q18.2-11). The land combat step (not to mention naval transport, RR, and ATR steps all allow moving units to violate FTC without consequence until the end of those steps.
If Harry forgot anything it's that the order of the retreat in that particular example doesn't matter. But now suppose there's one hex in Belgium and one in France available. The attacker can retreat the INF to France and the HQ to Belgium (given retreat priorities are respected) and then the INF dies due to FTC at the end of the land combat step, just like in the example in the rules.
No, this is not correct. This is covered in 11.11.5 and FAQ: 18.2-3.
If the INF is to be retreated into France then it must be allowed to do so by the FTC. So either they are both retreated into the Belgian hex. Or the HQI is first retreated into France...
RAC: 11.11.5 Active major powers
You can move a land unit controlled by an active major power into any hex controlled by:
• that major power and its aligned minors; or
• another active major power on the same side (or its controlled minor countries); or
• a major power or minor country it is at war with.
There are some exceptions:
• units can’t enter the home country of a non co-operating major power on the same side unless they satisfy the
foreign troop commitment rules (see 18.2); and
• minor country units can’t enter a hex controlled by another minor country aligned with their side unless they
satisfy the foreign troop commitment rules.
• units cannot enter a country controlled by another power on their side without permission of the owner.
Q: If answer to Q18.2-2 is yes, is a retreat
allowed into a hex where Foreign Troop
Commitments would be violated, even if
other hexes are available? The R result
says that valid ending hexes are those that
do not cause a stacking violation.
A: Or even if the answer to Q18.2-2 is no, no
you may not retreat into a hex a unit can
not move into.
Q: Is the failure to meet the Foreign Troop
Commitments an overstacking situation?
(who gets to decide what is lost?)
A: NO -- it determines whether a unit can
enter a country. Once in, it's OK even if
HQ moves out. Date 29/12/2007
Q: A stack of CW units in Belgium includes
an INF and an HQ-I. A German attack on
this stack achieves an ‘R’ result. The only
eligible retreat path is to a hex in
unconquered France and at the time of the
land combat, the Foreign Troop
Commitment for CW units in France is not
enough to allow the CW INF to stack in
France. Can the Germans retreat the CW
units into hexes in France in such a way
that because the foreign troop
commitment is not met and the INF unit
arrives in France before the HQ and is
thus destroyed for illegal stacking?
A: The CW units must be retreated if
possible, so in this example, the HQ must
be retreated first.
Intend attacking at dawn high water.