Jeschonnek called the German Embassy in Moscow on the morning 1 September 1939 to request that the Soviet Union keep the Minsk radio station identifying itself, so that German bomber pilots could use it for navigational purposes. The Polish Air Force and Polish Army offered fierce resistance though by 14 September the former offered only sporadic resistance. The campaign proceeded successfully, until a Polish counter-attack as the Battle of Bzura. The OKL was shocked into considering Operation Wasserkante—the codename for the bombing of Warsaw.