Taught by the folly of her stay-at-home strategy in 1866, Bavaria readily sent her large contingent beyond the Rhine; and all danger of a French irruption into South Germany was ended by the speedy massing of the Third German Army, some 200,000 strong in all, on the north of Alsace.

For the French to cross the Rhine at Speyer, or even at Kehl, in front of a greatly superior army (though as yet they knew not its actual strength) was clearly impossible; and in the closing hours of July the French headquarters fell back on other plans, which, speaking generally, were to defend the French frontier from the Moselle to the Rhine by striking at the advanced German troops.

At least, that seems to be the most natural explanation of the sudden and rather flurried changes then made.
It was wise to hide this change to a strategic defensive by assuming a tactical offensive; and on August 2 two divisions of Frossard's corps attacked and drove back the advanced troops of the Second German Army from Saarbruecken.

The affair was unimportant: it could lead to nothing, unless the French had the means of following up the success.

This they had not; and the advance of the First and Second German Armies, commanded by General Steinmetz and Prince Frederick Charles, was soon to deprive them of this position.
Meanwhile the Germans were making ready a weighty enterprise.