Casus belly for World War I was the murder of the Austrian successor to the throne archduke Franz Ferdinand and his spouse in Saraevo (Serbia) in June 15th (28th), 1914 by the Serbian student Gavrila Printzip. In July 10th (23th), 1914 Austria-Hungary, supported by Germany, presented to Serbia the ultimatum, containing among others the requirement of admission of the Austrian investigators to the territory of Serbia and introduction of a limited contingent of the Austrian troops.
Serbia expressed willingness to accept the basic requirements of the ultimatum, except for those two items, and to deliver the case on consideration of great powers or the Hague international tribunal. However on the 14th of July 1914 Austria-Hungary declared war to Serbia. Attempts to prevent the European war (England's offer on convocation of conference for resolution of the conflict, Nikolay's II telegrams to German emperor with proposal of intermediary, etc.) weren't successful.
In the evening of the 17th (30th) of July 1914 Russia declared the general mobilization. On the night of the 31st of July - the 1st of August Germany presented to Russia an ultimatum for cancellation of the decision on mobilization within 12 hours. S.D.Sazonov's repsponse was that cancellation of mobilization in such terms was impossible, but Russia was ready to continue negotiations for settlement of the conflict.On the 1 of August 1914 Germany declared war to Russia. On the 2nd-3rd of August France declared the support to Russia, and England - the support to France. In the evening of the 3 of August Germany declared war to France. On the 6 of August Austria- declared war to Russia.
The plan of the German command (after von Shliffen), developed in 1905, at the first stage of war presupposed the flash-like campaign on the Western front against France, and then actions against Russia, which armed forces were supposed to be held down by Austria-Hungary. Thus Germany assumed to avoid the war on two fronts and to finish it in less than 2-3 months.
The Russian General Staff considering the possibility of the Germany's defeat over France planned the simultaneous attacks on Berlin by the Northwest (under command of Y.G. Gilinsky) and Vienna by the Southwest fronts (under command of N.I.Ivanov). In order to rescue its allies Russian army assumed the offensive against Germany before the deployment of all armies and the mobilization were completed. On the 4th (17th) of August the first (under command of P.K.Rennenkampf) and the second (under command of A.V.Samsonov) Russian armies entered East Prussia, distracting German forces from the Western front where Germany, having broken the neutrality of Belgium, attacked Northern France.
In the beginning the onset of Russian armies developed successfully. On the 7th (20th) of August 1914 the 1st army inflicted a serious defeat to the 8-th German army commanded of Pritvits under Gumbinnen. In the meantime the 2nd army headed by general Samsonov advanced deep into East Prussia. Having taken the advantage of criminal inaction of Rennenkampf who hadn't organized the pursuit of hastily receding opponent, the new commander of the 8-th army general Paul von Gindenburg stopped the retreat and directed almost all the forces against Samsonov's army. Two corps of the 2nd army were surrounded and almost entirely destroyed. The rest of the army fell back to the Russian border. After that Gindenburg attacked the armies of Rennenkampf and by the 2nd (15th) of September had superseded them from East Prussia.
The situation in the Southwest front was more successful.During Galicia's offensive started on the 5th (18th) of August, Russian armies invaded Galicia, broke the Austrian-Hungarian army in counter fights, and on the 21st of August (the 3rd of September) the armies of general Brusilov occupied Lvov. Continuing the successful attack, Russian armies besieged the powerful Austrian fortress of Peremyshl and occupied a significant part of Bukovina. The threat of Russian intrusion to Silesia compelled the German command to shift a part of armies to the East front. On the 12th (25th) of September by order of the Russian Headquarters the attack was stopped, and armies were concentrated near Warsaw. From September to November 1914 the two largest battles - the Warsaw-Ivangorod and Lodzi - took place on the territory of Polland. Over 800 thousand soldiers from both sides took part in the battles.
At the end of November 1914 at the conference of the front leaders of the Russian army in Brest the decision to postpone the offensive till January 1915 was taken. One of the major causes of the decision was the lack of arms, shells, artilleries, an exhaustion of regular cadres. The losses of Russian army made 1,2 million of people - killed, wounded and captured. The war on the East front took a trench character. On the 20th of October (the 2nd of November) 1914 Russia declared war to Turkey, and Russian armies crushed the 3d Turkish army in Transcaucasia. The German-Turkish squadron suffered great losses on the Black sea.
Successful actions of Russian armies in the beginning of 1915 compelled Germany to relocate its forces to the Eastern front. The best cases were shifted from the Western front to form the shock army under command of general Makenzen. On the 19th of April (the 2nd of May) 1915 after intensive artillery preparation the German and the Austrian armies stroke a blow to Russian armies in Galicia, near Gorlitsa. Persistent fights lasted out for almost a month and a half. Russian armies slowly retreated. On the 21st of May (the 3rd of June) they left Peremyshl and then on the 9th (22nd) of June - Lvov. Russian armies had left almost all the territory won at the end of 1914, but the general breach of the Russian front in the given area wasn't a success. In July the German command made an attempt to encircle Russian forces near Warsaw. Fights in Poland went on for a month. On the 20th of July (the 2nd of August) Russian armies left Warsaw, then - Novogeorgiyevsk. The army was saved, but Poland was left to the enemy.
Developing its success, in August the German command launched the offensive in the directions of Kovensk, and then Vilno. The opponent had occupied Vilno and the significant part of Lithuania. Due to the actions of the Baltic fleet the breach of the German ships in the gulf of Riga didn't succeed.
By the end of 1915 Russian armies were dislodged from Poland, Lithuania, Galicia, parts of Baltic and Byelorussia; successes of the first months of war were brought to nothing. From the beginning of war the losses of Russian armies made 3,5 million people killed, casualty and captured. Allies did not render Russia any actual help.
Military failures of 1915 caused rearrangements in the supreme command structure of the Russian army: in June 1915 Minister of War V.A.Sukhomlinov, accused in treason, was dismissed. General A.A.Polivanov was appointed to his place. In August 1915 Nikolay II displaced his uncle Nikolay Nikolayevich from the post of the Commander - in - chief despite of his good military reputation and the authority in the army. At the same time the chief of the General Staff general Yanushkevich was also displaced. Nikolay II himself took the post of the Commander - in - chief, and the post of the chief of the General Staff occupied general M.V.Alexey - the son of a common soldier, who had made military career due to his abilities, one of the future heads of white movement. Efficient actions of M.V.Alexeev as the Commander-in-chief of the Northwest front ruined the enemy's plans to surround and annihilate the Russian army in Poland in July - August 1915.
In 1916 the German command made a decision of the defensive strategy on the Eastern front, intending to defeat its opponents in the West. Great success was achieved by Russia in operations against Turkey. On the 6th of February 1916 the armies of the Caucasian front took the possession of Erzerum and in April 1916 seized Trapezund, thus having forwarded 250 kilometres far inland Turkey.As a result of the whole military campaign of 1916 the countries - participants of Entente with the conclusive influence of the armed forces of Russia had grasped the strategic initiative in the struggle against the German-Austrian block.