Complex sentence: Major Calloway then asks Martins to help capture Lime, and Martins agrees, asking for Anna's safe conduct out of Vienna in exchange; but when Anna learns this, she refuses to leave.
Split into simpler sentences: Major Calloway then asks Martins to help capture Lime, and Martins agrees, asking for Anna's safe conduct out of Vienna in exchange. However, when Anna learns this, she refuses to leave.