Under the terms of the Treaty of Versailles, signed on June 28 1919, Germany was forced to surrender much of its territory. One of these was the province of Alsace-Lorraine, which had been annexed by the German Empire in 1871, and which the Germans were forced to return to France. In addition, Germany also lost parts of West Prussia and Silesia when these were granted to Poland. Thirdly, Germany also lost the Hultschin District which was given to Czechoslovakia. (See the reference link provided).
In total, historians have estimated that Germany forfeited 13 percent of its European territory (amounting to 27,000 square miles) as a result of the Treaty of Versailles. This affected approximately 6.5 to 7 million people, around one-tenth of the German population. (See the second reference link provided).