For travel to the United States:

The USDA and CDC have no specific requirements for bringing a cat into the country, but it is prudent to have a certificate of good health from a local veterinarian.

You should also get an official “third country veterinary certificate” from a state veterinarian in order to properly exit Montenegro. Your local vet can help arrange this.

You must also check your home state’s requirements (there is a link on the USDA page); most states now require a rabies shot for animals over 3 months of age.

And don’t forget to check your airline(s) for any specific requirements they have, or that your pet may need, to transit through the EU.

https://www.aphis.usda.gov/aphis/pet-travel/bring-pet-into-the-united-states/pet-travel-cats-into-us

https://www.cdc.gov/importation/bringing-an-animal-into-the-united-states/cats.html

NOTE: Microchips are now routinely implanted along with the rabies vaccine in Montenegro. Unfortunately, these microchips cannot be “read” by the US scanning system, so you should plan to have a US chip implanted when you arrive home. For travel, please make sure the animal is wearing ID tags with several reachable phone numbers on them, in case the animal escapes while en route.

Airline considerations for US travel:

Please note that animals must be 4 months/16 weeks old in order to fly from Europe to the United States. Make sure that any estimates of age on medical records comply with this requirement or the animal will be denied transport out of Europe.

If you already have a flight booked out of Montenegro, Croatia, or Serbia with a layover in the EU, please note that not all European airports will allow animals to transit through them. In Germany, for example, Frankfurt does but Munich does not. You may need to change your reservation to a route that allows you to take an animal with you.