This last example introduces us to a fresh consideration, of very great importance.  It is not only that every progressive community has had to solve, in one way or another, the problem of securing permanent concert of action without sacrificing local independence of action; but while engaged in this difficult work the community has had to defend itself against the attacks of other communities.  In the case just cited, of the conquest of Greece by Rome, little harm was done perhaps.  But under different circumstances immense damage may have been done in this way, and the nearer we go to the beginnings of civilization the greater the danger.  At the dawn of history we see a few brilliant points of civilization surrounded on every side by a midnight blackness of barbarism.  In order that the pacific community may be able to go on doing its work, it must be strong enough and warlike enough to overcome its barbaric neighbours who have no notion whatever of keeping peace.  This is another of the seeming paradoxes of the history of civilization, that for a very long time the possibility of peace can be guaranteed only through war.  Obviously the permanent peace of the world can be secured only through the gradual concentration of the preponderant military strength into the hands of the most pacific communities.  With infinite toil and trouble this point has been slowly gained by mankind, through the circumstance that the very same political aggregation of small primitive communities which makes them less disposed to quarrel among themselves tends also to make them more than a match for the less coherent groups of their more barbarous neighbours.  The same concert of action which tends towards internal harmony tends also towards external victory, and both ends are promoted by the co-operation of the same sets of causes.  But for a long time all the political problems of the civilized world were complicated by the fact that the community had to fight for its life.  We seldom stop to reflect upon the imminent danger from outside attacks, whether from surrounding barbarism or from neighbouring civilizations of lower type, amid which the rich and high-toned civilizations of Greece and Rome were developed.  When the king of Persia undertook to reduce Greece to the condition of a Persian satrapy, there was imminent danger that all the enormous fruition of Greek thought in the intellectual life of the European world might have been nipped in the bud.  And who can tell how often, in prehistoric times, some little gleam of civilization, less bright and steady than this one had become, may have been quenched in slavery or massacre?  The greatest work which the Romans performed in the world was to assume the aggressive against menacing barbarism, to subdue it, to tame it, and to enlist its brute force on the side of law and order.  This was a murderous work, and in doing it the Romans became excessively cruel, but it had to be done by some one before you could expect to have great and peaceful civilizations