A year later, Ivan took part in the defense of the unification of the Principality of Bulgaria and the province of Eastern Rumelia as volunteer in the Student's Legion during the Serbo-Bulgarian War.

[2] On 14 January 1886, he was admitted to the Military School in Sofia and, due to his high grades, the young cadet was assigned to study in the artillery section.

Bulgaria responded by declaring war on Romania on 1 September and initiating the planned first major offensive of the Central Powers during the Romanian Campaign.

[6] General Kolev led his division in the First Battle of Cobadin, but despite their efforts the Bulgarians were forced to suspend their attack due to the losses and exhaustion they had suffered from the almost three weeks of continuous fighting.

This allowed the Romanians and Russians for the first time since the beginning of the campaign to gain the initiative and prepare a major offensive aimed at the destruction of the Bulgarian Third Army.

To achieve this they concentrated a force of 124 infantry battalions, 89 artillery batteries and 31 cavalry squadrons that were supposed to attack and penetrated the Bulgarian lines while the Romanian Third Army crossed the Danube at Flămânda and advanced in their rear.

For some ten days during the heavy fighting, General Kolev remained on the very front of the battle often in the trenches where he was exposed to severe rain and low temperatures that began affecting his health.

Breaking through the main defensive line, General Kolev pushed back the Romanians and Russians towards the important village of Topraisar and threatened their flank.

[10] This and the success of the left wing of the Central Powers forces made the Allied positions untenable and they soon began a general retreat.

[11] Following his orders, General Kolev began their pursuit almost immediately, defeated the withdrawing Romanians and Russians at Constanţa, and entered the vital port city.

In November, the cavalry division had to undergo some structural changes ordered by the new commander of the Third Army, General Stefan Nerezov, that were opposed by Kolev.

[10] His favorite horse, Pirin, with which he fought throughout the four months of the Dobruja Campaign, became a legend in the army and continued serving until 21 May 1925 when it was retired due to its old age (it was 15 years old at that time).