Complex sentence: In the meantime the 1st Proletarian Corps, the main partisan formation in Serbia, continued with reinforcing and developing its forces and with seizing positions for the assault on Belgrade: on 18 September Valjevo was taken, and on 20 September Aranđelovac.
Split into simpler sentences: In the meantime, the 1st Proletarian Corps, the main partisan formation in Serbia, continued with reinforcing and developing its forces and with seizing positions for the assault on Belgrade. On 18 September Valjevo was taken, and on 20 September Aranđelovac.