Complex sentence: Erfurt became part of the Kingdom of Prussia in 1802, part of the First French Empire in 1806 as Principality of Erfurt, and was returned to Prussia in 1815 after the Napoleonic Wars.
Split into simpler sentences: Erfurt became part of the Kingdom of Prussia in 1802. The city became part of the First French Empire in 1806 as Principality of Erfurt, and was returned to Prussia in 1815 after the Napoleonic Wars.