Layover in Frankfurt - customs? security?

It is not enough time especially if you use two different airlines. I would allow 3 hours.

I may be out of touch but I don't think Croatia is part of the EU so some sort of customs is probably required. A one hour connection anywhere these days is insane (even with just carry on bags). How many on-time arrivals have you experienced lately? Don't think you'd have to re-clear security. Usually once you clear customs there's a luggage transfer area (within the security zone) where you shlep your bags for the next flight (same airline or not). Totteling along to the next gate is within the security zone as well. Give yourself 3 hours (really).

Thank you both - I figured that was probably the case. Wishful thinking. The other option appears to be a 12 layover and another stop. But the trip is a ways away, connections may change yet.

I would definitely allow more than an hour. Frankfurt does have 2 terminals and if you arrive in one and depart from the other one, you will have to go through security again. Always expect delays, not to do so is just wishful thinking. Go ahead and plan a long layover here as it is easy to come into the city and see the sights. Takes about 15 min. on the subway.

Please note that the minimum connection time in Frankfurt is 45 minutes. This works if you incoming flight is on time. You should be able to check your bags all the way through to your final destination - so won't need to deal with them in FRA.

I don't believe that Croatia is part of the Schengen area - at least it wasn't last year when we traveled there through FRA. Hence, the flights should arrive in Term 2 (i.e. the B set of gates) and this is where most of the LH and UA flights to the US leave from though some UA flights also leave from the C gates. If changing terminals, you will have to go trough security (twice if traveling to the US).

Sorry I'm not knowledgable about the other American carriers (DL, NW, US, etc.) and what terminal they leave from in FRA.

Croatia is definitely not within the Schengen area, so you don't have to clear customs and passport control. I just flew from Croatia to Washington DC via Vienna, and was surprised at how simple it was to transfer--all of the non-passport control gates are within a minute's walk of each other, and there are special security screenings at each individual gate, so that only took a minute or so as well. As it turned out, I had a couple of hours between flights, but I could have easily done the transfer in a half hour or less.

I think Vienna has the reputation of being very easy to make these non-passport control connections, but I would be surprised if Frankfurt didn't have a very similar system. I would recommend that you make sure that your flights are all code-shared with the same airline, regardless of who is actually piloting the plane. That way you only have one airline responsible for your ticketing, baggage, etc.