Liberty Leading the People (La Liberté guidant le peuple) by Eugène Delacroix commemorating the July Revolution of 1830. This woman is commonly referred to as "Marianne" and is considered a symbol of the French fighting for freedom.
Apart from fictional tales about the Amazons and other legendary warrioresses (as Camilla of the Aeneid), there are no reliable traces in the European history of classical times of women having fought in battle. In time, things changed and, in our age, the more than 100,000 women who fought in the Red Army during the 2nd world war were a crucial factor in the Soviet victory.
But when did women start fighting in Europe? There are scattered reports here and there of women fighting, say, with Napoleon's army and even earlier than that: think of Joan D'Arc, who fought for the French against the English in the early 15th century.
How far back can go in history and find fighting women? I think that the earliest report is from Paul the Deacon who describes a battle between the Wandals (later known as the Vandals) and the Winnili (later known as the Longobards). Here is an excerpt from the "Historia Langobardorum" (book I):
At this point, the men of old tell a silly story that the Wandals coming to Godan (Wotan) besought him for victory over the Winnili and that he answered that he would give the victory to those whom he saw first at sunrise; that then Gambara went to Frea (Freja) wife of Godan and asked for victory for the Winnili, and that Frea gave her counsel that the women of the Winnili should take down their hair and arrange it upon the face like a beard, and that in the early morning they should be present with their husbands and in like manner station themselves to be seen by Godan from the quarter in which he had been wont to look through his window toward the east. And so it was done. And when Godan saw them at sunrise he said: "Who are these long-beards?" And then Frea induced him to give the victory to those to whom he had given the name. And thus Godan gave the victory to the Winnili. These things are worthy of laughter and are to be held of no account. For victory is due, not to the power of men, but it is rather furnished from heaven.It may be wholly fictional, but if a battle between the Longobards and the Vandals really took place, it must have happened before the Longobards moved into Italy, so not later than the 6th century AD, possibly much earlier. And if it there really was such a battle, this report can be understood as meaning that the Longobard women didn't just show up on the battlefield, they fought! And from what we know of the Longobard and their assertive queens (think of Theodolinda), it looks plausible. It looks also consistent with some archaeological evidence of female warriors buried in Northern Europe in early Middle Ages.
So Paul the Deacon gives us perhaps the first recorded (albeit fictionalized) case of women fighting in a battle in Europe. They have come a long way from those times!