Complex sentence: He was an Ambassy Officer and/or Ambassador in Russia from 1759 to 1763, Ambassador in Portugal from around 1765 to 1778, Ambassador in London in 1778 but leaving in 1779 because of disputes with the British on the issues of the American War of Independence.
Split into simpler sentences: He was an Ambassy Officer and/or Ambassador in Russia from 1759 to 1763, Ambassador in Portugal from around 1765 to 1778, Ambassador in London in 1778. The following year he was forced to leave the position because of disputes with the British on the issues of the American War of Independence.