In the summer of 1914, Dr. Paču was the first to receive an Austrian protest note on the occasion of the Sarajevo assassination, which he handed over to absent Pasic. On 23 July, Ministry of Foreign Affairs Baron Gizl presented the Austro-Hungarian ultimatum to the Serbian authorities, including Pachu, with a 48-hour deadline. The instruction from Foreign Minister Count von Berchtold was to leave the country immediately and the Serbian government should not accept all the points of the ultimatum. The reply was delivered by the Serbian Prime Minister Pasic five minutes before the deadline expired on 25 July. As the Serbian government did not accept all the points, Gizl declared that diplomatic relations were broken off and left on a train half an hour later. On 28 July 1914, Austria-Hungary formally declared war with Serbia. During the summer of 1915, while Lazar Pachu worked as Finance Minister of the Serbian government in Nis, he became ill. In June, after doctors removed a stone from his bladder, he fell into bed. Due to the need for immediate recovery after the operation, he lived a month in Vrnjci. In Vrnjacka Banja, he died two months later, in October 1915, and there he was buried. His body was later transferred to the New Cemetery in Belgrade.