A series of makeshift houses were detonated on Saturday in an annual commemoration of the Battle of Mlawa -- one of the opening battles in the German Invasion of Poland -- which took place in early September 1939. The country's 20th infantry division, part of the Polish Modlin Army, held out against the German forces for three days. The reenactment took place in the city center and on the historical battlefield, attracting several thousand visitors. The event usually involves the display of military memorabilia and a picnic lunch serving typical army-fare pea soup.