After the commencement of the Polish-Bolshevik War, as a company commander he participated in liberating Dyneburg, Żytomierz and in the 1920 Polish Expedition to Kiev.

Remaining on active duty after World War I, he was promoted to major and posted to the 1st Polish Infantry Regiment, as a battalion commander.

Shortly before the outbreak of World War II, he was made commander of the 51st Giuseppe Garibaldi Rifle Regiment within the 12th Infantry Division on the eastern fringes of Poland (Kresy Wschodnie).

He was interned in October 1939, but fled several weeks later from a camp and reached France via Hungary, where he joined the newly-forming Polish Armed Forces in the West.

In September of that year, he was smuggled back to occupied Poland as the first emissary of the Polish government-in-exile, under the nom de guerre "Nil" which he had chosen for himself.

Shortly before the collapse of the Warsaw Uprising on 28 September 1944, he was promoted to the rank of brigadier general with an order from the Supreme Commander Kazimierz Sosnkowski.

He was also nominated for future command of the NIE Organisation, which was formed from the cadre of the AK with the intention of resisting the new Polish Stalinist government.

Released in 1947, he returned to new Poland ruled by the communist Polish Workers' Party government and the increasingly repressive Ministry of Public Security.

Fieldorf was accused by prosecutor Helena Wolińska-Brus of being a "fascist-Hitlerite criminal" and having ordered an execution of Soviet partisans while serving in the AK.

[9] An appeal to a higher court failed, and the family's plea for a pardon was denied by then the communist leader Bolesław Bierut who refused to grant clemency.

The executioner and one of the guards approached the condemned […] I went to see the warden afterwards, and then by my own hand I prepared the protocol of the execution.General Fieldorf's body was never returned to his family, and was buried in a location which remains unknown.

In 2009, an article in a British Telegraph newspaper suggested that Fieldorf was buried in a mass grave in a Warsaw cemetery, together with the remains of 248 other murdered Polish non-communists.

A 1956 report issued by the communist authorities concluded that Wolińska-Brus had violated the rule of law and was involved in mock investigations and show trials that frequently resulted in executions.

The charges against her were initiated by the Institute of National Remembrance, which claimed Wolińska-Brus was an "accessory to a court murder", which is classified as a Stalinist crime, and is punishable by up to 10 years in prison.

Speaking about other individuals who have had complicity in the court-sanctioned murder (that is of fabricated evidence) of her father, Maria Fieldorf-Czarska said: I can neither allow the investigation [into the murder] of my father to be closed, nor to allow the following individuals to escape justice: the Deputy Director of Justice at the General Prosecutors' Office, Alicja Graff, the Prosecuting Attorney, Wiktor Gattner, and the Interrogator, Kazimierz Gorski.

I would want us, Poles, so much, to be able to choose our own examples to follow, and not those promoted by others.In 2009 a historical drama movie entitled Generał Nil based on Fieldorf's life premiered in Poland to generally positive reviews.