Complex sentence: The Toungoo command seriously considered attacking Theinni--an army commanded by Thado Minsaw was already in the neighboring Thibaw--but decided not to open another front while Lan Na remained a threat to southern Shan states.
Split into simpler sentences: The Toungoo command seriously considered attacking Theinni as Thado Minsaw's army was already in the neighboring Thibaw. But they decided not to open another front while Lan Na remained a threat to southern Shan states.