 She started her 40 @-@ year reign when her father , Emperor Charles VI , died in October 1740 . Charles VI paved the way for her accession with the Pragmatic Sanction of 1713 and spent his entire reign securing it . Upon the death of her father , Saxony , Prussia , Bavaria , and France all repudiated the sanction they had recognised during his lifetime . Prussia proceeded to invade the affluent Habsburg province of Silesia , sparking a nine @-@ year conflict known as the War of the Austrian Succession , and subsequently conquered it . Maria Theresa would later unsuccessfully try to reconquer Silesia during the Seven Years ' War . 
