 Frederick II of Prussia (who became Maria Theresa's greatest rival for most of her reign) promptly invaded and took the affluent Habsburg province of Silesia in the seven-year conflict known as the War of the Austrian Succession.In defiance of the grave situation, she managed to secure the vital support of the Hungarians for the war effort.Over the course of the war, despite the loss of Silesia and a few minor territories in Italy, Maria Theresa successfully defended her rule over most of the Habsburg empire.