Complex sentence: In 661, the Tang forces advanced towards Goguryeo and surrounded its capital of Pyongyang for several months until in February 662, when it had to withdraw from the campaign due to the harsh winter conditions.
Split into simpler sentences: In 661, the Tang forces advanced towards Goguryeo. The Tang army besieged Pyongyang, Goguryeo's capital, for several months until February 662, when it had to withdraw from the campaign due to the harsh winter conditions.