The first action of the Franco-Prussian War took place on 4 August 1870. This battle saw the unsupported division of General Douay of I Corps, with some attached cavalry, which was posted to watch the border, attacked in overwhelming but uncoordinated fashion by the German 3rd Army. During the day, elements of a Bavarian and two Prussian corps became engaged and were aided by Prussian artillery, which blasted holes in the defenses of the town. Douay held a very strong position initially, thanks to the accurate long-range fire of the Chassepots but his force was too thinly stretched to hold it. Douay was killed in the late morning when a caisson of the divisional mitrailleuse battery exploded near him; the encirclement of the town by the Prussians threatened the French avenue of retreat.
Is there an answer to this question (If it cannot be answered, say "unanswerable"): What was the date of the first action of the Franco-Prussian War?
4 August 1870