After graduation, he returned to his hometown to farm, though according to one source, he was hired by the Cape Colony government at a young age as a plunge dip inspector of cattle.
In 1895, he moved to the South African Republic, where he and his friend Koos Jooste made a name for themselves as record-setting champion cyclists on the Pretoria-to-Cape-Town race.
De Wet and Jooste criticized the government for not stationing troops in Colesberg and advocated an expedition to the northwestern districts of the Cape to foment anti-colonial rebellion and thereby cut off the British connection to the South.
The surrender of Gen.Piet Cronjé at Paardeberg on 27 February 1900, and the fall of Bloemfontein on 13 March 1900 obligated Republican forces to retreat quickly south of the Orange River.
After a difficult trip through German South West Africa, he joined the commanders in the Cape in October 1901 and served on the staff of Gen. Jan Smuts, until a serious injury put him out of commission once more.
However, the latter Boer generals would only visit the German capital afterwards, and therefore the company went to the Netherlands where Neser rented them rooms with money borrowed from Cape loyalist Jan du Plessis.
With the outbreak of World War I, De Wet enlisted in the Freikorps on 9 September 1914 in Windhoek, hoping among other things to free his homeland South Africa from British rule.
Armed with ammunition, uniforms, materiel, and military support from the German government in Windhoek, the Freikorps joined the rebels in the Northwest Cape in their siege of Upington.
De Wet journeyed with the Boer troops from Windhoek to Nakop, serving as captain and commander of the Freikorps regiment sent to join Gen. Maritz there, but soon resigned and was replaced by Cmdt.
[7] Quickly and poorly trained for numerous battles against the much better equipped UDF, the Freikorps ultimately failed to gain the upper hand in Cape Colony.
After the rebellion was suppressed and the slight Freikorps incursion into the Northwest Cape ended, Botha's army had little trouble defeating the Schutztruppe and capturing Windhoek on May 9 without opposition.