Hey Matthew,
If you're fighting against just one country, the defending country's interceptor wings will respond to defend the country until they're all destroyed or depleted.
If you're fighting against a country that is in a federation - nearby interceptor wings from federation members will ALSO join to help defend.
Fighting vs. 1 country, you're probably taking shots with 500 interceptrs (1 wing) responding at a time.
If you're fighting a country that has federated nations nearby, you might be taking shots with 1500 (3 wings) interceptors defending/responding at a time.
The wing size increased from 400 to 500 interceptors, which made a surprisingly big difference in how effective fighting them can be.