Zass took part in the War of the Sixth Coalition with the Russian army in 1813–1814, in German campaign of 1813 after swearing allegiance to the Tsar, being of Germanic-Baltic descent.

He fought at Dresden, Kulm and Leipzig, was awarded the insignia of the Military Order and promoted to cornet, then participated in the battles in the 1814 campaign in north-east France at Brienne, Bar-sur-Aube, Arcis-sur-Aube, Metz, and Fère-Champenoise, and ended the fighting under the walls of Paris.

Having studied the military traditions of Caucasian natives and having mastered the tactics of mountain warfare, he tried to seize the initiative and began to carry out regular preventive measures, based on data received from numerous scouts.

His fearlessness and incredible awareness of the affairs of the Caucasians brought him an otherworldly status among the Circassians, who called Zass "Shaytan" (Satan).

In 1834, Zass sent a report to Georg Andreas von Rosen, a general in the Russian army, detailing his campaign into Circassia.

After a victory, he would usually burn several villages and seize cattle and horses to show off, acts which he proudly admitted.

On his initiative, new fortifications began to be laid (one of them is called "Zassovsky") and new Orthodox Slavic villages were arranged, which was the beginning of the creation of the Labinskaya line.

Using various tricks and cutting-edge technology, he was able to convince and deceive the Circassians into thinking that he had various magical abilities, including being bulletproof and able to turn gunpowder into gold.

As a result, when he attacked, the Circassians believed they had no chance and suffered a significant loss of morale; knowing Zass's cruel methods, they mostly attempted to save their lives rather than fighting.

[20] On 18 August, the Russian army burned the residency of Hajji Tlam, a Circassian elder from the Abdzakh region, and killed his family.

General Zass sent Circassian commander Jembulat Boletoqo word in October 1836 that he wanted to make peace.

On the same night, when the Circassians were celebrating their oppressor's death, the suddenly "resurrected" Zass launched a raid that destroyed two villages.

Tsar Nicholas I of Russia, in March 1849, recruited a Russian army, composed of about 8,000 soldiers,[22] to assist Franz Joseph against the unrest in Hungary.

[22] But as they crossed the Southern Carpathian mountain passes (along the border of Transylvania and Wallachia), they were met by a large Hungarian revolutionary army led by Józef Bem, a Polish-born General.

[25] Emperor Alexander II, wishing to honor the military exploits of Zass, called him back into service and promoted him to general of cavalry, commanding him to be listed in the Caucasian Army with the right to live wherever he wanted.

Zass died at the age of 86 in Šķēde Manor, Courland Governorate, Russian Empire (now Saldus Municipality, Latvia).

[29] The Decembrist Nikolai Ivanovich Lorer said that Zass cleaned and boiled the flesh off the heads after storing them under his bed in his tent.

[40][41][42][43][44][45][46] Zass worked with another German officer in the Russian army named Georg Andreas von Rosen during the genocide against the Circassians.

Zass wrote letters to Rosen proudly admitting he ordered Cossacks to slaughter Circassian civilians.

[47][48] In addition, it was recorded that Zass dismembered Circassian corpses, hid them as ornaments and sent them abroad to be used as test subjects.

He developed a plan to strengthen the left bank of the Belaya River, to create powerful strongholds for the Russian army.