The response can, and historically has, come from any nation, not just the one the ship is registered in. For instance in the last (Somali) attack before this,

The response can, and historically has, come from any nation, not just the one the ship is registered in. For instance in the last (Somali) attack before this, a Maltese flagged tanker was boarded, and a Spanish warship arrived the next day and retook the ship.