In 1073 Hildebrand assumed the tiara as Gregory VII., and declared a war that lasted more than forty years against the Empire.

At its close in 1122 the Church and the Empire were counterposed as mutually exclusive autocracies, the one claiming illimitable spiritual sway, the other recognized as no less illimitably paramount in civil society.

From the principles raised by Hildebrand and contested in the struggles of this duel, we may date those new conceptions of the two chief powers of Christendom which found final expression in the theocratic philosophy of the _Summa_ and the imperial absolutism of the _De Monarchia_.

Meanwhile the Empire and the Papacy, while trying their force against each other, had proved to Italy their essential weakness.
What they gained as ideas, controlling the speculations of the next two centuries, they lost as potentates in the peninsula.

It was impossible for either Pope or Emperor to carry on the war without bidding for the support of the cities; and therefore, at the end of the struggle, the free burghs found themselves strengthened at the expense of both powers.
Still it must not be forgotten that the wars of Investitures, while they developed the independent spirit and the military energies of the Republics, penetrated Italy with the vice of party conflict.