The last thirty years of the nineteenth century saw the European balance of power at its most perfect: five great powers (with a doubtful sixth), each able to maintain its independence, none strong enough to dominate the others. The irreconcilable antagonism between France and Germany, and the equally irreconcilable, though less persistent, antagonism between Austria-Hungary and Russia in the Balkans, prevented the creation of any preponderant combination. The balance of power took on the appearance of a natural law, self-operating and self-adjusting; Europe enjoyed the longest period of peace known in modern times; and the powers turned their energies outwards to ‘imperialist’ expansion. All acquired empires; some at their own backdoor, the others overseas.

The Franco-Prussian War, which broke out in July 1870, created this exceptional balance. It began as a French attempt to arrest the progress of German unity; instead it freed Europe from the shadow of French predominance without putting German predominance in its place. It was the last war fought solely in Europe and confined to European great powers. It was indeed confined to two powers. This was unexpected. Great Britain was genuinely neutral once Belgium was secured. But Austria-Hungary prepared to intervene on the French side, though only after French victories. Russia first talked vaguely of threatening Austria-Hungary into neutrality; then, with equal vagueness, planned to compete with her for French favour. These calculations came to an abrupt stop as the campaign developed. The first battles on the frontier went against France. On 3 September the main French army was defeated and compelled to surrender at Sedan. Napoleon III became a prisoner. The French empire was overthrown, and the Republic proclaimed in Paris.