Z39 served with the 6th Destroyer Flotilla her entire German career, which she spent escorting transports in the Baltic Sea, laying mines, and bombarding land forces.

Throughout her German service, the ship laid numerous barrages (explosives concentrated over a wide area) of mines in the Baltic Sea and bombarded Soviet forces several times.

In the last months of the war, Z39 helped escort steamships that were evacuating German soldiers and civilians from Eastern Europe to Denmark.

Following the end of World War I Germany signed the Treaty of Versailles, which put strict limits both on the size and displacement of warships that she could possess.

During the Interbellum, the period between the first and second world wars, the average size of Allied ships and their armaments in almost all warship categories grew substantially.

[3] Although German heavy destroyers matched British light cruisers in armament, they were much less seaworthy and had far worse facilities for control and use of their guns.

Due to the clear advantage her enemies had, Raeder remarked that the Kriegsmarine could not hope to win, and thus the only course for them was to "die valiantly".

[9] During World War II, destroyers served essentially the three basic functions they had in World War I: to act as screening ships to defend their fleets from those of an enemy, to attack an enemy's screening ships, and to defend their fleet from submarines.

[18][19] These gave the ship a rated power of 70,000 shaft horsepower (52,000 kW), and a top speed of 38.5 knots (71.3 km/h; 44.3 mph).

[20][12] Z39's sensor suite housing included a FuMO 21 radar that was placed on the ship's bridge and four FuMB4 Sumatra aerials on the foremast searchlights.

The small number of German shipyards forced the Kriegsmarine to prioritize construction, inexperienced naval engineers, and the lack of workers.

[15] After her commissioning, the ship began minelaying operations in the Skagerrak and the Kattegat until March 1944 when she was transferred to Reval off the Gulf of Finland.

[28] One such operation lasted from 13 to 14 April, in which Z39, two other destroyers, and six minelayers laid the "Seeigel 6b" mine barrage south of Suur Tytärsaari.

[37] On 10 April she and T33 (a torpedo boat) escorted the German destroyer Z43, which had sustained damage from both mines and bombs,[38] to Warnemünde and Swinemünde.

[39] From 1944, German surface ships were called upon to provide support for Army Group North along the Baltic Sea coast.

This tactical use of cruisers, destroyers, and torpedo boats was difficult in the restrictive waterways of the Baltic, but despite these difficulties, it justified the continued existence of the surface fleet.

[40] From the spring of 1945 to near the end of the war, the surface forces of the Kriegsmarine became almost entirely focused upon resupplying and supporting garrisons along the Baltic Coast.

After March 1945 the Kriegsmarine embarked upon the task of evacuating hundreds of thousands of civilians and soldiers from the east ahead of the Soviet forces, which were rapidly pushing westward.

On 15 April, Z39 with two other destroyers and four torpedo boats escorted the German steamships Matthias Stinnes, Eberhart Essberger, Pretoria and Askari to Copenhagen with 20,000 refugees.

[39] At some unknown point after the war ended, Z39 sailed with a mixed German and British crew to Wilhelmshaven and then to Plymouth, England on 6 July 1945.

[45] She left England on 30 July, and arrived in Boston on 7 August where on 14 September, after extensive trials, the destroyer was commissioned into the US Navy as DD-939.

[39][47] After arriving in Casablanca in January 1948, she sailed to Toulon, where DD-939 was redesignated as Q-128, and was later cannibalized for her parts, which were used to repair the French destroyers Kléber (ex-Z6 Theodor Riedel), Hoche (ex-Z25), and Marceau (ex-Z31).