Petri declared the war against Lithuanian kingdom in 1212 AD. There were three major competing powers: royal troops of Lithuanian King, rebels led by power vassals and intervents headed by Izaslav (commander of Vladimir army). In the spring of 1212 AD more than ten thousand-strong army of Vladimir kingdom crossed the borders of Lithuania. Initially, intervents besieged castles of Polotsk in order to form a bridgehead exposed the capital of Lithuania -
Kernave. Lithuanian royal army, for its part, invaded territory of White Rus and placed the garisson of Minsk under siege. Minsk is a fortified keep located at te border with Lithuania, that has a strategic influence on the following course of war. Rebels besieged Kernave, whose army numbered almost 20 000 men as well.
After capture of Polotsk Izaslav marched toward White Rus to relieve the siege of Misk. The first battle occurred on 22 April 1212 AD. Army led by Izaslav defeated 7000 men army of Lithuanian king. The road to Kernave was free. At the same time, rebels took the castle of Kernave by storm. old capial of Lithuanian kingdom has been sacked and looted by rebels. After victory of Minsk battle, army of Izaslav moved toKernave and lay siege to the city. After a siege which lasted less than 10 months, Kernave was recaptured. The city was looted by Vladimir troops in december 1212 AD, Queen of Lithuania was captured. It was the second pillaging the capital for 2 years.
Despite Queen of Lithuania, father of the King was captured by Izaslav. Both prisoners were sent to Moscow in the dungeon of Petri. Petri forced them to convert into Suomenko faith. He hoped for convertion of entire Lithuania because father of Lithuanian king was a heir. Unfortunately, he was slained futher in one battle, so "convertion" plans would never work. News about fall of the capital Kernave came to the rebels. They turned around troops and moved to the capital again. Battle between rebels and Izaslav armies occured on 25 August 1215 AD, Izaslav lost the battle... Petri was so angry he decided to execute one of prisoners... Queen of Lithuania.
The majority of the Izaslav fighters retreated to Riga, small holding conquered by republic of Osel (vassal of Vladimir Kingdom). Stock of the army was replenished, forces were consolidated and Izaslav again marched toward Kernave. The third siege of Kernave started in February of 1217 AD. The city was properly encircled the city to force a quick surrender, which included cutting off the water supply. After a siege of 5 months, Kernave surrendered to the Vladimir's troops. Buildings and agricultural properties were demolited, citizens were looted and condemned to absolute poverty and misery, Military fortifications were destroyed.
Izaslav pillaged villages and small cities to undermine enemy's strength and damage moral fiber of Lithuanian troops. He defeated two enemy armies )one is rebellious, another is royal), marching to save Kernave. Up to the winter of 1217 AD, the capital of Lithuania was completely in ruins, most of baltic holdings were looted. Meanwhile, Lithuanian kinf, Feodor II, moved to novgorod region to counterattack. He could raise almost 6 000 men and besieged Pskov. Izaslav also moved to Pskov region, he could trap enemy army of Feodor at wetlands of Pskov. The battle was extremely ferociou, but Izaslav managed to dismantle Feodr's force. One half of his army was destroyed, another part - dispersed.
Treaty of Pskov
In the following months Feodor and Petri signed peace agreement. DejureNovgorod holdings (Novgorod, Pskov, Neva) were transfered to Vladimir kingdom, Feodor II was forced to pay an indemnity of 400 ducats. Petri could seriously undermine power of Lithuanian Kingdom. New Lithuanian dynasty was extemely weak, Fedor lost civil war and lowered significantly crown authority. Kingdoms of Denmarks and Pomerania also separated from Lithuania.