After he had passed his examination in diplomacy in 1897, he was assigned to the Prussian mission at Hamburg but quickly switched again to Rome, where he advanced to the position of Second Secretary (legation counsellor).

After a short interlude with the German mission at The Hague, he returned as First Secretary to the embassy in Rome in March 1901, where he stayed until 1906, when he was transferred to the Foreign Office in Berlin.

"[3] The illusion of peace masked debates over aims of colonial annexation and supremacy in Africa, negotiated neutrality for other states to facilitate the invasion of France, and attempts to compete with British sea power.

In fact the fortnight's delayed response to Austria's commencement of hostilities gave Jagow the opportunity to blame Russia for starting the war.

On 24 July the British thought he was "quite ready to fall in with suggestion as to the four Powers working in favour of moderation at Vienna and St Petersburg"[11] But he was already ill and exhausted from his exertions, acknowledging that Serbia was the victim of bullying.

[12] He was a member of the Foreign ministry team that denied a British offer for a Five Power Conference, having already agreed to Moltke's plan two days before Austria's declaration against Serbia on 28 July 1914.

When the Ottoman Empire declared war on the Entente in November 1914, Jagow directed Leo Frobenius to try to persuade the government of Abyssinia to also join the Central Powers.

[16] Instrumental was Under Secretary Zimmerman, one of Bethmann's governmental supporters, who ran Agent Parvus in Constantinople: the aim was "the complete destruction of Tsarism and the dismemberment of Russia into smaller states".

Yet determined to continue the fight in Galicia, when Jagow received President Wilson's offer of an international peace congress negotiations, American mediation was flatly refused.

By 30 October Jagow had developed the idea that Courland and Lithuania should be annexed in return for Austrian-Poland "chaining" the Dual Monarchy's destiny to Germany's.

Nonetheless Jagow was responsible for the Longwiy-Breiy Plan to occupy the plateau that overlooked the city of Verdun made possible by Baron Romberg's visit to Berlin.

His meddling with Ushida in St Petersburg got him 'discarded' too for attempting a separate peace with Russia on Germany's behalf; he even confessedly admitted on 17 May 1916 that the "whole swindle ceases to matter".

Jagow wrote a defence of German policy entitled Ursachen und Ausbruch des Weltkrieges (“Causes and the outbreak of the World War”), published in 1919.