▁August ▁military ▁operations ▁began ▁on ▁the ▁Polish ▁border . ▁Lib au ▁was ▁bomb arded ▁by ▁a ▁German ▁cru iser ▁on ▁ 2 ▁August ▁and ▁on ▁ 5 ▁August ▁Mont en eg ro ▁declared ▁war ▁on ▁Austria - H ung ary . ▁On ▁ 6 ▁August , ▁Austria - H ung ary ▁declared ▁war ▁on ▁Russia ▁and ▁Ser bia ▁declared ▁war ▁on ▁Germany ; ▁war ▁began ▁between ▁Mont en eg ro ▁and ▁Germany ▁on ▁ 8 ▁August . ▁The ▁Battle ▁of ▁St all up ön en ▁( 1 7 ▁August ) ▁caused ▁a ▁minor ▁check ▁to ▁the ▁Russian ▁invasion ▁of ▁East ▁Pr uss ia ▁and ▁on ▁ 1 2 ▁August , ▁Britain ▁and ▁France ▁declared ▁war ▁on ▁Austria - H ung ary , ▁as ▁Aust rian ▁forces ▁crossed ▁the ▁Save ▁and ▁seized ▁Sh ab atz . ▁Next ▁day , ▁Aust rian ▁forces ▁crossed ▁the ▁Dr ina ▁and ▁began ▁the ▁first ▁invasion ▁of ▁Ser bia . ▁The ▁Battle ▁of ▁Cer ▁( B attle ▁of ▁the ▁J ad ar , ▁ ▁began ▁and ▁the ▁Battle ▁of ▁G um bin nen ▁in ▁East ▁Pr uss ia ▁took ▁place ▁from ▁ ▁On ▁ 2 1 ▁August , ▁Aust ro - H ung arian ▁forces ▁with d rew ▁from ▁Ser bia . ▁The ▁Battle ▁of ▁T ann enberg ▁ ▁began ▁in ▁East ▁Pr uss ia ▁and ▁in ▁the ▁Battle ▁of ▁Gal icia ▁ ▁the ▁First ▁Battle ▁of ▁Kra ś nik ▁was ▁fought ▁in ▁Poland ▁from ▁ ▁Sh ab atz ▁was ▁ret aken ▁by ▁Ser bian ▁forces ▁and ▁the ▁last