A dog traveling in the cargo hold are subjected to more stressful conditions than those traveling in the passenger cabin with its owner. Tranquilizing a dog is not recommended. Dogs should be fully recovered from any kind of surgery, including neutering and spaying. This trait is very important. Pets that are easily provoked into aggressive behavior towards humans and other canines are better left at home, for yours and everyone else's sake.
Service dogs are exempt.
If your dog is prone to car and air sickness, talk to your vet or a dog trainer about resolving it. Finally, let's face it. Some dogs, especially puppies, are better left at home with a pet-sitter or boarding kennel instead of being subjected to the stress of air travel, loud mechanical noises and unfamiliar conditions. You, the caring dog owner and your vet are the best judges of your dog's endurance. For foreign travel, you'll need to contact the foreign embassy in the U.S. of your destination country.