There have been many wars fought within and between European nations . Whether you consider wars between nations to be nationalistic is a matter of opinion.
Notwithstanding, the following wars were started (or purportedly started) for unambiguously nationalistic reasons since in the 20th century:
- The Balkan Wars fought to liberate ethnic minorities from Ottoman rule - 224,000 killed
- The First World War initiated by Austria/Hungary to avenge the assassination of Archduke Franz-Ferdinand killed by an ultra-nationalist Serbian organisation - 8 million battle deaths perhaps as many as 20 million civilian deaths through consequential famine. If the 1918 flu pandemic is considered a consequence of the war (and at the very least its global spread was heavily dependent on the movement of servicemen across the world) this adds another 50-100 million deaths.
- The Spanish Civil war, a conflict that started in a political plot by the fascist Falange and CEDA political parties to overthrow the republican government, and when the coup failed (17-18 July 1936), the generals affiliated to those parties rose with their regiments in an act of war against the government. The conflict had an estimated casualties of around 500.000 estimated deaths, a similar amount of injured or amputees, and over 300.000 (the estimate in that document is previous to the 2011 and 2012 investigations) executions after the war.
The war was waged by the most fascist and conservative sectors of Spain, Recieved support from Germany and Italy, and served as testing grounds for WW2 (Panzer 1 and 2, Strategical and civilian bombing, Shock infantry charges, urban warfare, etc).
- World War II in Europe in 1939 (some argue that the war started in 1937 in China) was launched by Germany over demands on Poland to cede German speaking parts of Poland - an estimated 50-85 million people were killed worldwide by direct or indirect violence: these deaths were not limited to Europe or European nations, however, 27 million were Soviets (again not all of the European as the USSR was both a European and Asian nation) and Germany lost 5.3 million service personnel.
- Yugoslav Wars were ethnic/nationalistic conflicts prompted by the dissolution of Yugoslavia - figures are hard to come by but certainly tens and possibly hundreds of thousands were killed.
If this is the basis then 20 million is an extremely conservative estimate.