1915 was to be a year of heavy defensive fighting in the West and strong offensives in the East and Southeast.
In the east Hindenberg almost annihilated the opposing Russian army in the battles in Masurian Lakes.
Cavalry and infantry entered Lithuania and Kurland and in May the Germans smashed the Russian positions on the Gorlice-Tarnow front. Mackensen's troops pushed the Russians from one sector to the next and the Russian Carpathen army fell to pieces. The fortress of Przemyl was retaken and the positions at Lemberg fell. The troops then swung northwards to attack the parts of the Russian Army in Poland from the rear. All along the Eastern Front the German army rolled forward pushing the Russians back in a series of bloody fights and battles. Fortresses like Nowo-Geogiwsk and Grodno fell in just a few days, rivers like the Narew and Njemen crossed after bloody fighting. The advance was swift but due to exhausted troops and stiff Russian resistance the Germans missed their goal of encircling and destroying the Russian troops in Poland.
In the meantime the Serbs had inflicted a series of defeats on the Austrians. As the Serbs stood in the way of the Axis route down to Turkey and the access it provided to the sea, they were an enemy that needed defeating. The Bulgarian army joined the Axis and attacked from the East. From the north over the Save and Donau rivers came an Austrian army with German divisions under Mackensen. The two rivers were crossed with little problem but then they came into the mountains, where the Serbs defended the few roads into their homeland with dogged determination.
A mixed French-British Army had landed in Salonika to come to the aid of the Serbs but it was too little, too late. The Axis managed to push the Serbs aside and reach the southern border. The Allied Salonika army remained a threat, however, while the Serbs retreated into the Albanian mountains.
The beginning of the year in the West saw a few small tactical victories for the Germans on the left flank in the Soissons and Münster areas but already in the last weeks of February the first of four great allied offensives challenged the German positions. The first offensive was the "Winterschlacht in der Champagne", the second the British Offensive at Neuve Chapelle. In autumn the third and fourth efforts would take place with another offensive in Champagne coupled with the battle by La Bassee and Arras that were together meant to smash the German front once and for all. All these efforts failed with a terrible loss of life.The Germans spent the year on the defensive although a gas attack in Ypres did allow them to take some land and the famous Hartmannsweilerkopf was retaken from the French after bloody fighting.