Answer based on context:

The Russians and the Japanese both contributed troops to the eight-member international force sent in 1900 to quell the Boxer Rebellion and to relieve the international legations under siege in the Chinese capital, Beijing. Russia had already sent 177,000 soldiers to Manchuria, nominally to protect its railways under construction. The troops of the Qing Empire and the participants of the Boxer Rebellion could do nothing against such a massive army and were ejected from Manchuria. After the Boxer Rebellion, 100,000 Russian soldiers were stationed in Manchuria. The Russian troops settled in and despite assurances they would vacate the area after the crisis, by 1903 the Russians had not established a timetable for withdrawal and had actually strengthened their position in Manchuria.

What were the 177,000 soldiers guarding?
railways under construction