Hah! The only Encyclopedia Brown mystery I remember some thirty plus years nigh. Don’t remember the Manassas part, but definitely the sword and the “First Battle” part.
There’s that too; the answer is that it is if that’s what you want to call it. There’s no real solid line dividing between when something is an offensive and when it’s a counteroffensive. Not that google is the arbiter of anything, but googling “Ardennes Counteroffensive” and “Ardennes Offensive” both return plenty of results both referring to the 1944 one. “Ardennes Offensive” produces numerically many more, but that doesn’t really mean anything other than it is probably the more commonly used one of the two. Results for “Ardennes Counteroffensive” are hardly lacking in authority leading to such places as the Eisenhower Presidential Library, the Imperial War Museum and the US Army’s Center of Military History.
At the end of the Great War the British set up a Battle Nomenclature Committee to assign names to the various battles on the Western Front; It eventually published its conclusions as The official names of the battles and other engagements fought by the military forces of the British Empire during the Great War 1914-1919 and the Third Afghan War 1919: report of the Battles Nomenclature Committee as approved by the Army.
Some, like the Third Battle of Ypres, commanded more-or-less universal acceptance in British historiography, others, like the Battle of Albert, are now used by nobody at all.
the ‘von Rundstedt offensive’ was also used for it at the time, although von Rundstedt actually had very little to do with it other than the operations orders going out under his nominal signature.