Croatia to Frankfurt connection to US

Since this is a non-Schengen to non-Schengen flight there is no need for immigration, and you won't see your bags in Frankfurt. There should also be no need for security, apart from whatever extra security is needed for US flights.

We have flown through Frankfurt on Lufthansa several times from Marseilles to the U.S., and we have had short connections, anywhere from 45 minutes to just over an hour. We always made our connection, but I admit it was a bit stressful, especially with the 45 minute layover. We did go through passport control. Is your flight home on one continuous ticket from Croatia to the U.S.? If so, your baggage will be checked through all the way to the U.S. You don't go through security or customs. I remember one time our fight was delayed leaving Marseilles, and when we arrived in Frankfurt, we only had about 20 minutes. We walked very, very fast (I couldn't run because of arthritis problems). The airline had apparently notified our flight to Boston that we were coming, so they waited for us. The staff at the gate applauded when they saw us. We boarded about 5 minutes before take-off. Obviously, I hope this never happens again.:-) As I understand it, if you miss your connecting flight, it is the airline's responsibility to put you on the next flight home. (As long as both flights are on one ticket.)

Since this is a non-Schengen to non-Schengen flight

HUH? Did Frankfurt secede from Germany? Is this info out of date?--https://www.schengenvisainfo.com/schengen-visa-countries-list/
It shows Germany as a Schengen member, but not Croatia.

Another key element is one ticket vs two tickets ( i.e. separate bookings.)

Since this is a non-Schengen to non-Schengen flight

This is true so passenger should be able to transit without immigration.

This is true. Delta just emailed me about it earlier today - not at Frankfurt, but at CDG, but same scenario. I'm going Seattle to Paris to Tel Aviv. Delta sent me a note that says since my flight is originating from and departing to a non-Schengen country, there is no need to clear passport control. I will have to do security though. At least for my flight on Delta single ticket, bags are checked through.

Since this is a non-Schengen to non-Schengen flight

HUH? Did Frankfurt secede from Germany? Is this info out of
date?--https://www.schengenvisainfo.com/schengen-visa-countries-list/
It shows Germany as a Schengen member, but not Croatia. Another key
element is one ticket vs two tickets ( i.e. separate bookings.)

When did the US join Schengen?

That website is a private website that in no way provides any officiall information. However, they are correct that Germany is in Schengen but Croatia is not (even if they will probably join soon).

Assuming this is all one ticket, you will not go through passport or customs in Frankfurt. Therefore, no need to retrieve bags. You only walk from your arrival gate to your departure gate.

joe32f, an airport concourse that contains people who have not passed through an immigration gate is not "in" the country where the airport is located. There aren't as many Transit Lounges as there used to be in airports. But those big glass walls separating arrivals from other sections of the airport are there for a reason.

The OP has not indicated whether this is one ticket or an Itinerary they put together. If on two tickets, and they check bags, then yes they will need to go through immigration, collect bags, check bags, security, exit immigration, then on to their gate....1.5 hours is way less than I would feel comfortable with, could be stranded in Frankfurt.

If on one ticket, bags go through, just head to your gate, 1.5 hours should be fine, if not, the airline will still get you home.

Thank to those who have helped clear my muddled head.

This part of the original posting

Will we be able to retrieve our checked bags and complete the transfer and security process within that timeframe?

Made me think they have separate tickets and in retrieving their luggage would leave the 'pristine' area and need to go through the full entry/exit process. My interpretation of what OP is doing may be incorrect.

And I read the word connection in the title and assumed that it was a true connection, i.e. one ticket.