Complex sentence: Under President Ulysses S. Grant, he was Assistant Secretary of State in 1869-71 and again in 1873-74, after being secretary of the commission which concluded the Treaty of Washington in 1871 which produced the settling the Alabama claims.
Split into simpler sentences: Under President Ulysses S. Grant, he was Assistant Secretary of State in 1869-71 and again in 1873-74. Between times he was a secretary of the commission which concluded the Treaty of Washington in 1871, to create a tribunal to settle the Alabama claims.