A "sitzkrieg" is a stalemated war, or one where the two sides are doing more facing off than fighting.
The term comes from World War II, and refers specifically to the western nations' initial response to the German assault. The Nazi war machine attacked Poland in 1939, using a style of warfare they referred to as Blitzkrieg - German for "lightning war" - characterized by the concentration of overwhelming firepower on single points by highly mobile mechanized and airborne forces. England and France reacted by immediately declaring war on Germany, but then did little to respond to the Germans' new style of fighting. The French, in particular, waited behind the Maginot Line for the Germans to attack, believing that their fortifications would enable them to rout the German forces. In later years, this was referred to as the Sitzkrieg, pseudo-German for "sitting war" - derisively, as the lackadaisical French army was outflanked by German forces and swiftly and decisively crushed.
There's a wonderful photo taken by a Time magazine photographer that perfectly illustrates both the poor response to the German threat, and the reason that there was such general contempt for it afterwards. The photograph shows a lonely section of the Ardennes forest, no human habitation or construction in sight, with a single French soldier on lookout. The soldier, dressed in full battle gear, surveys his surroundings with a keen gaze - with his rifle across his knees, as he sits on a wooden chair. The contrast between this tranquil image and the films and images of the German asssault is striking, and humorous in a grim sort of way.