During the times of king Daniel's reign, the Galician lords were allied with their Polish neighbours against a common threat, the Lithuanian tribes that often raided the neighbouring lands for loot and plunder. However, in 1255 (or the previous year) Shvarn married an unnamed daughter of Mindaugas, since 1253 the first (and only) king of Lithuania. This allied him to Lithuania and together the two rulers undertook numerous military campaigns against the Kingdom of Poland. Already in 1255 they raided Lublin, in 1262 a major campaign against Masovia was started. Shvarn and Treniota captured the city of Płock and besieged Shvarn's brother-in-law, Siemowit I of Masovia in Jazdów (modern Warsaw). In the end Siemowit was killed by Shvarn's troops and his son Konrad II was taken prisoner. The Polish relief force did not arrive in time and was later defeated in a battle at Długosiodło on August 5, 1262.