Complex sentence: The intervention in Etruria had been very costly to Spain and Maria Luisa secret dealing with Murat had been seen as against the interest of her native country, she was considered a foreign Princess aiming at gaining a throne for her son.
Split into simpler sentences: The intervention in Etruria had been very costly to Spain and Maria Luisa secret dealing with Murat had been seen as going against the interest of her native country. She was considered in Spain as a foreign Princess aiming at gaining a throne for her son.