Denmark was waiting for French Forces to land to declare war on Prussia. The French had a navy infantry division ready for that, but it was diverted to the army after the border defeats and surrendered with Napoleon at Sedan.
The French plan initialy considered an offensive in southern Germany to cut the South German states from Prussia and join with Austrian forces.
The Germans kept a couple of corps on the North Sea Coast, and some troops on the Austrian border, at the opening of the war, until the situation in France made it unecessary.
There had been talks between France Italy and Austria, and those were hesistating. Bismarck however moved first and asked them to declare themselves immediately. Italy and Austria backed down.
At the appropriate he got out of his hat an old proposal to give Belgium to France as a compensation for not gaining anything in a war France was not involved in (that was the diplomacy of the time), so France remained isolated.
Overall the German unification under Prussia owes a lot to Bismarck's crafy diplomacy.