Complex sentence: Wendi of Sui, displeased with challenge of Goguryeo and continued small scale raiding into Sui's northern border by Goguryeo troops, sent the diplomatic papers in 596 that amounted to an ultimatum to the Korean dynasty.
Split into simpler sentences: Wendi of Sui was displeased with the challenge from Goguryeo, which continued small scale raiding into Sui's northern border by Goguryeo troops. Wendi sent diplomatic papers in 596 that amounted to an ultimatum to the Korean dynasty.