At the outbreak of World War I he discontinued his studies and received officer's training in the Drużyny Strzeleckie. He moved to Kraków, where on August 16, 1914 he joined the Polish Legions. On September 30 his unit was dispatched to the front. He served with distinction in the II Brigade of the Legions, and held the posts of platoon, company and battalion commanding officer. After the Oath Crisis, together with many other members of the Polish Legions of Russian citizenship, he joined the 2nd Infantry Regiment of the Polish Auxiliary Corps, and then since February 16, 1918 he served in the Polnische Wehrmacht. On October 30 he left the ranks of his unit and the following day he joined the Polish Army, even before Poland officially declared her independence. Until July 13, 1919, he served as a platoon commander in the 8th Legions' Infantry Regiment, and then he commanded the entire regiment during the Battle of Lwów and the Polish-Ukrainian War. After the end of hostilities and a brief period in Belarus and Lithuania, Smorawiński became the organizer and commanding officer of a newly formed 9th Legions' Infantry Regiment. He returned to his previous unit in August of that year and remained its commander until October.