Complex sentence: The conservatives were alarmed by the continuous increase of the socialists' influence since 1899, with the climax in 1917, without the offsetting control of Russian administration; the social democrats had to be halted before they were able to markedly alter the political structure.
Split into simpler sentences: The conservatives were alarmed by the continuous increase of the socialists' influence since 1899, which reached a climax in 1917. Since the Russian administration could no longer control the effectiveness of the social democrats, the conservatives felt it necessary to act before the social democrats were able to markedly alter the political structure.