My spouse, with whom I had been living separately for several years, has just passed away. He had no children, and I have learned that he left all his assets to the Red Cross. Is this legal ?

No. Despite your years of separation, you still retain the status of "entitled spouse" (Article 732 of the Civil Code). In the absence of children, the law provides that you are entitled to at least 1/4 of his estate (Article 914-1 of the Civil Code). Therefore, you are entitled to request from the universal legatee an indemnity known as "reduction" for your share of the inheritance (Article 921 paragraph 1 of the Civil Code).