If you have an empire of multiple countries and one country is attacked, will the defensive air wings of the other countries in the empire defend the country under attack?
If you form an alliance for your bloc and endorse the defense clause, I believe so. Also if you air-transport a wing(s) from country A to country B (vice versa) both wings from both countries will defend together. In effect doubling your defense against a single attack. You can triple etc. this way also.