After the February Revolution, he left the Russian Army and joined the Polish 1st Corps, being formed in Russia as part of the Entente forces.

The Polish Army badly needed experienced officers, and Kopański joined the 1st Uhlans Regiment, with which he fought in the battles of Przemyśl, Gródek Jagielloński and Lwów.

At the end of hostilities, Kopański remained in the army and fought in the opening stages of the Polish–Soviet War in the Lida and Wilno areas, in the forces of Colonel Władysław Belina-Prażmowski.

After the Peace of Riga, Kopański was demobilized and allowed to finally finish his engineering studies, this time at the Warsaw University of Technology.

In October of that year, he was dispatched to Paris, where he commenced his studies at the Ecole Superieure de Guerre, one of the most notable military academies of the time.

In May 1930, he became the commanding officer of one of the battalions within the 6th Heavy Artillery Regiment, stationed in Lwów, but resumed his post in the General Staff a year later.

However, due to fast pace of German advance, the headquarters had to be evacuated further southwards, through Młynów, Kołomyja and Kosów, to the town of Kuty, where it was to organize the defense of the so-called Romanian Bridgehead.

It was not until 5 April 1940 that Kopański was finally given command of the Polish Carpathian Brigade, being formed in Homs on the border between French-held Syria and Lebanon.

The unit was composed mostly of Polish soldiers who were able to escape prisoner of war camps in Hungary and Romania and make it to Allied-controlled territory, much like Kopański himself.

After the siege was lifted on December 10, the brigade joined British forces in their pursuit of the withdrawing Italo-German armies and fought in the Battle of Gazala.

He organized that division out of his former unit and newly arrived soldiers of the Polish II Corps of General Władysław Anders, who had been liberated from Soviet gulags and then evacuated to Persia and Palestine.

Shortly before the invasion of Italy commenced, Kopański was withdrawn to London on 21 July 1943, where he was appointed Chief of Staff of the Commander-in-Chief of the Polish Armed Forces in the West.

This was a paramilitary organization designed to allow Polish veterans who were unwilling to return to communist-dominated Poland to find employment and homes in the west.

On 23 November 1971 the Communist authorities of Poland declared their decision to deprive Kopański of citizenship null and void; this however was never made public.