Osowiec Fortress (Polish Twierdza Osowiec) is a 19th-century fortress located in north-eastern Poland, built by the Russian Empire. It saw heavy fighting during World War I when it was obstinately defended for several months by its Russian garrison against German attacks.
The fortress was built in the years 1882-1892 as one of the defensive works to protect the western borders of Russia against Germany, and continuously modernised afterwards to cope with advances in heavy siege artillery. In 1889-1893, military engineer Nestor Buinitsky took an important part in the creation of the fortress. It was located on the river Biebrza about 50 km from the border with the German province of East Prussia, in the one place where the marshlands of the river could be crossed, hence controlling a vital chokepoint. The extensive marshlands and bogs that surrounded it made attacks upon it difficult. The strategic Białystok - Ełk (Lyck) - Königsberg (Kaliningrad) rail line also ran through the fortress and crossed the Biebrza river there. The fortress saw heavy fighting during the beginning of World War I in the eastern front from September 1914 until the Russian Army abandoned it in August 1915. In the interwar years the fortress was used by the Polish Army. During the German invasion of Poland in 1939 it was bypassed and did not see much fighting.
Today, some parts of the fortress are accessible to tourists, especially the parts within the boundaries of Biebrza National Park. The visitor information center of the park is located in Osowiec-Twierdza, a small settlement located within the boundaries of the fortress. Other parts of the fortress still belong to the Polish Army and access is restricted.
1st German Assault - September 1914
In September 1914, the fortress was put under siege by parts of 8th German army - 40 infantry battalions that attacked immediately upon arriving. By 21 September, having a huge advantage in numbers, the Germans troops were able to push back Russian field defenses to the point where German artillery could fire upon the fortress itself. At the same time German command reinforced their forces with 60 artillery pieces of calibers up to 203mm; however, these pieces could only start firing on 26 September 1914. Two days afterwards the Germans decided to try a frontal assault of the fortress, but it was cut down by a fierce fire from Russian artillery. The next day the Russians made two flanking counter-attacks that forced the Germans to quickly relocate artillery to a safer place, although this put their guns too far from the fortress to fire on it.
2nd German Assault - February to March, 1915
On 3 February, 1915, German forces attempted a 2nd assault on the fortress. A long and hard battle was fought for the control of 1st line of field defenses. The Russian forces were able to hold off numerically superior forces for 5 days in shallow trenches. On 9 February, Russian command decided to pull back all the forces to the 2nd field defense line that had deep trenches and established machine gun placements.
During the next two days Russian forces gave no ground, but the retreat from the first line allowed German artillery to start firing on the forts on February 13. The caliber of German heavy siege artillery varied from 100 mm up to 420 mm. The cannons fired in groups of 360, and every four minutes 360 explosions rattled the fortress. Throughout the week-long artillery barrage, 250,000 shots were fired from the heavy guns and about one million rounds were fired from the light artillery pieces.
The Russian Central Command, thinking that they were asking the impossible, requested the fortress last at least 48 hours to allow the evacuation of civilians to be completed. The Osowiec Fortress lasted for half a year.
The Russians suffered heavy losses from the artillery barrage, which was strongest from 14 to 16 February and from 25 February to 5 March 1915. Multiple hits inside the fortress and the collapse of many buildings made movement between different parts of the fortress almost impossible. Even in those hellish conditions the Russian artillery managed to destroy two out of the four heavy mortars and forced the German commanders to pull the remaining two back.
Because the 2nd Russian field defense line was never breached Germans were forced into positional warfare on this part of the front until the beginning of July.
3rd German Assault - July to August, 1915
At the beginning of July, under the command of field-marshal von Hindenburg, German forces began a full frontal offensive on the fortress. Forces included 14 battalions of infantry, one battalion of sappers, 24-30 heavy siege guns, and 30 batteries of artillery equipped with poison gases. The Russian defenders had 500 soldiers of the 226 Earth division and 400 militia men.
The Germans waited until 6 August for the right wind conditions. At 4 am, at the same times as regular artillery started their bombardment, German forces used poison gases against the defenders. Thinking that all of the defenders were dead, fourteen battalions of Landwehr - at least 7000 infantry men - began advancing. When German infantry reached the first line of defense, they encountered what was left of 13th company of the 226th Zemlyansk regiment (about 100 men). Being unprepared for resistance and seeing the bloody clothing the remaining defenders wore (Russian soldiers were coughing blood up because of the effects of the poison gases) put the Germans in a state of shock and caused them to break and run. The five remaining Russian guns subsequently opened fire on the retreating Germans. European papers afterwards called this «The attack of the dead men».
Fifteen days later, Russian commanders finally pulled back the last remaining soldiers from Osowiec and retreated to new positions.
- Хмельков С. А. (Khmelkov, S.A.) (1939). Борьба за Осовец (Struggle for Osovets) (in Russian). Moscow: Государственное военное издательство наркомата обороны СССР.
- Perzyk Bogusław (2004). Twierdza Osowiec 1882 - 1915 (in Polish). Warszawa: Militaria Bogusława Perzyka. ISBN 83-907405-1-6.