Complex sentence: By 1870 three of the important lessons of the Austro-Prussian war became apparent: through force of arms, a powerful state could challenge the old alliances and spheres of influence established in 1815.
Split into simpler sentences: By 1870 three of the important lessons of the Austro-Prussian war had become apparent. The first lesson was that, through force of arms, a powerful state could challenge the old alliances and spheres of influence established in 1815.