The Romans soon violate the treaty: Priscus…

Emperor Wen orders his fifth and youngest son, Yang Liang (assisted by the co-prime minister Gao Jiong), and Admiral Zhou Luohou, to conquer Goguryeo with an army and navy totaling three hundred thousand.

Yang Liang's army faces the early rainy season when it reaches Goguryeo in August.

The unseasonably heavy rain make the army's progress almost impossible and hampers the transport of provisions.

Constant attacks by Goguryeo forces and illness inflict heavy casualties.

Coming to the conclusion that the army cannot achieve the objective on its own, Yang decides to combine with Zhou's naval fleet and proceed.

Zhou's navy also encounters their own challenges, contending with rough seas, losing many ships despite staying close to the coastline.

Whenever they anchor, Goguryeo detachments are present to attack the Sui sailors.

The fleet engages in a battle against a Goguryeo fleet totaling fifty thousand led by Admiral Gang Isik, presumably on the present Bohai Sea.

Already weakened by ambushes and nature's onslaught, the Sui fleet suffers a devastating loss and Yang Liang is forced to retreat.

King Yeongyang sends an embassy to Daxing; Emperor Wen accepts a peace agreement with Goguryeo.

He claims a hollow victory, as the Sui Dynasty has lost nearly ninety percent of its army and navy during the disastrous campaign.

The Avars lift the siege of Tomis on March 30, 598, since Comentiolus has led an army of largely inexperienced soldiers over the Haemimons and is moving along the Danube up to Zikidiba, near modern-day Medgidia, just thirty kilometers (nineteen miles) from Tomis.

Priscus, for reasons unknown, does not pursue the Avars and aid Comentiolus.

The latter is forced to evade to Iatrus, where his troops are nonetheless routed and have to fight their way south over the Haemus range.

The Avars, after ravaging many cities in the Balkan peninsula, utilize this victory and advance to Drizipera near Arkadiopolis, between Adrianople and Constantinople, where a large part of their army and seven sons of the Avar Khagan are killed by the plague.

Comentiolus is temporarily relieved of his command and replaced by Philippicus, whereas …

…Maurice summons the Circus factions and his own bodyguards to defend the long walls east of Constantinople.

For the time being, Maurice manages to buy off the Avars, and in the same year, concludes a peace treaty with Bayan, the Avar Khagan, explicitly allowing Roman expeditions in Wallachia.

The Romans used the remainder of the year to reorganize their forces and analyze the causes of failure.

Maurice makes peace with Agilulf, conceding northern Italy.

Pope Gregory I negotiates a truce, ending thirty years of Lombard terror.

Agilulf expands the Lombard Kingdom by occupying Sutri and …

…Perugia.

Tardu declares himself to be ruler (khagan) of the united Turkic Khaganate (east and west) in 599.

His new status is not recognized widely in the empire.

Probably to persuade the kurultay, he begins a campaign against China, but unlike the Turks of the eastern part of the Khaganate, his target is too far away and his army suffers intensely from the poisoned waterwells during the long expedition through the steppes.

Finally, he has to retreat without having forced serious combat.

This defeat is disastrous for him.

After a rebellion of his subjects, he will disappear from the scene (probably killed) in 603 or 604.

The Hungarian plain between the Danube and Tisza rivers has become the center of the Avar empire, which reaches its peak at the end of the sixth century, encompassing the area between the Adriatic and the Baltic Sea and between the Elbe and Dnieper rivers.

Maurice refuses to pay the ransom to free twelve thousand imperial soldiers taken prisoners by the Avars.

Their leader Bayan I orders the execution of the prisoners: all are slaughtered.

The emperor’s failure to buy back the captives destroys Maurice's popularity with the imperial troops in the Balkan Peninsula.

In summer 599, Maurice sends his generals Priscus and Comentiolus to the Danube front against the Avars.

The generals join their forces at Singidunum and …

…advance together down the river to Viminacium.

The Avar khagan Bayan I meanwhile, learning that the Romans have determined to violate the peace, crosses the Danube at Viminacium and invades Upper Moesia, while he entrusts a large force to four of his sons, who are directed to guard the river and prevent the Romans from crossing over to the left bank.

In spite of the presence of the Avar army, however, the East Roman army crosses on rafts and pitches a camp on the left side, while the two commanders sojourn in the town of Viminacium, which stands on an island in the river.

Here Comentiolus is said to have fallen ill or to have mutilated himself so as to be incapable of further action, but Priscus assumes command over both armies.

Unwilling at first to leave the city without Comentiolus, Priscus is soon forced to appear in the camp, as the Avars are harassing it in the absence of the generals.

A battle is fought which costs the Romans only three hundred men, while the Avars lose four thousand dead.

This engagement is followed by two other great battles in the next ten days, in which the strategy of Priscus and the tactics of the Roman army are brilliantly successful.

In the first, nine thousand of the Avars and their Slav allies fall, while the second is fatal to fifteen thousand, of whom the greater part, and among them the four sons of the khagan, perish in the waters of a lake, into which they are driven by the Roman swords and spears.

Priscus subsequently pursues the fleeing khagan and invades the Avar homeland in Pannonia, where he wins another series of battles on the banks of the river Tisza, deciding the war for the Romans and ending, for a time, the Avar and Slavic incursions across the Danube.

The onset of winter prevents the imperial army from capitalizing on its victory.