Most countries (the U.S. included) follow fairly similar standards when it comes to taxation of stock options - if they have a vesting period, taxation is propo

Most countries (the U.S. included) follow fairly similar standards when it comes to taxation of stock options - if they have a vesting period, taxation is proportioned accordingly with where you spent that vesting period. So, if they vested over 4 years and you spent 2.5 years of that in the U.S., it doesn't matter that you moved back home for a year to exercise. 2.5-years worth of stock options are considered U.S.-sourced and you owe taxes to USA when exercising them.