If you left the States and then found out your ex used your credit cards and you have a collection agency after you can they take legal action and can they arrest you?

Left the states as in the US?If you are militarythat is irrelevant, because they know where you are.Be that as it may, your liability depends on whose name the card(s) were in, what your divorce decree stated, etc. Although credit card companies don't really care what is stipulated in a divorce settlement. They want their money, period. They can sue you, garnish wages, bank accounts and take other actions to get payment. You cannot be arrested. There are no debtor prisons, fortunately. If there were they would be filled, with decent people who have fallen on hard times.