Seems like a relatively small state. How were they able to equip and mobilize their army so quickly after just having been a vassal
Just to address some misconceptions: Serbia was independent since 1878, and between then and WWI was engaged in three wars: against Bulgaria in 1885, against the Ottoman Empire in 1912 and again against Bulgaria in 1913. But even before that as a vassal state, Serbia had a relatively massive army since the second rule of prince Mihailo (1861-1868).