The fragile Austro-Hungarian defenses faltered at the first push of the Romanian covering forces.
Two regiments from the 15th Brigade of the 8th Division seized Kézdivásárhely (Târgu Secuiesc) on 29 August, the Romanians arriving in time to destroy the last train leaving the town.
The Austro-Hungarians did manage to blow up a critical railroad viaduct near the border.
Although the Romanians had prepared a special commando unit under Lieutenant Ilie Șteflea to cross the border three hours before the start of the invasion and secure the viaduct, the 14th Romanian Infantry Regiment jumped the gun and crossed the border along with the commandos.
Having lost the element of surprise, the Romanians alerted the Austro-Hungarians, who blew up the railroad viaduct.
In early September, the Romanians reached Csíkszereda (Miercurea Ciuc), capital of Csik County.
There, they temporarily halted their offensive and dug in on the east side of the Olt River.
Further to the north, the Austro-Hungarians gave up Bélbor (Bilbor) after running out of ammunition.
One brigade kept the Romanians largely east of Borszék (Borsec) while gendarmes held their own in the Kelemen (Călimani) Mountains.
For the most part, however, Austro-Hungarian soldiers along the border limped towards the Transylvanian interior, accompanied by hordes of German and Hungarian refugees.
For instance, over two thirds of the population of Csíkszereda left the town.
However, as events would unfold, such behavior would prove exaggerated.
In many places, the ethnic-Hungarian civilian population attacked small scattered groups of Romanian soldiers.
A fact not merely admitted, but praised in the Hungarian press of the time, such as the newspaper Pesti Napló.
In response, the Romanians nowhere took to reprisals, beyond fighting and disarming the civilians who put up active resistance.
Wherever the Romanian Army advanced, proclamations were published promising safety and protection to all inhabitants, regardless of nationality.
No damage was done by the Romanian armies to the property of civilians, regardless if said property belonged to ethnic-Hungarians (Székelys) or ethnic-Germans (Transylvanian Saxons).

On 8 September, the Romanians announced themselves in possession of Maroshévíz (Toplița), Csíkpálfalva (Delnița) and Gyergyószentmiklós (Gheorgheni).
The Romanian North Army crossed the Olt River on 7 September.
The 7th Division, after pushing aside the 19th Honvéd Brigade, continued its advance through the mountains towards Székelyudvarhely (Odorheiu Secuiesc).
Having shouldered aside the 16th Honvéd Brigade at Gyergyószentmiklós, the 14th Division reached the eastern edges of the salt-mining town of Parajd (Praid) on 11 September.
On 13 September, the Romanians had driven the 1st Landsturm Cavalry Brigade from the heights near Praid.
By late September, the I Reserve Corps under the command of the German General Curt von Morgen, had joined the Austro-Hungarians in their fight against the Romanian North Army.
The Corps succeeded in slowing the Romanians down.
Von Morgen's I Reserve Corps comprised two infantry divisions: the 89th German and the 71st Austro-Hungarian.

On 28 September, the Romanian North Army renewed its offensive.
Despite stubborn Austro-Hungarian resistance, it continued to inch forward throughout the following week, advancing to within six miles of Szászrégen (Reghin) and securing the heights above Parajd by early October.
In early October, the Romanian General Staff issued orders for the retreat of the North Army, due to the events unfolding in the region of Dobruja.
General Arz ordered his tired forces to pursue the Romanians, but the exhausted Austro-Hungarians moved slowly, giving the Romanians an uncontested run to the border, where they settled into defensive positions prepared before the war.
The pace of the pursuit is exemplified by the situation of  Székelyudvarhely.
This town, capital of Udvarhely County, was taken by the Romanians on 21 September.
It was retaken by the Austro-Hungarians on 8 October, however the Romanians were already gone.
Having
The fragile Austro-Hungarian defenses faltered at the first push of the Romanian covering forces. Two regiments from the 15th Brigade of the 8th Division seized Kézdivásárhely (Târgu Secuiesc) on 29 August, the Romanians arriving in time to destroy the last train leaving the town. The Austro-Hungarians did manage to blow up a critical railroad viaduct near the border. Although the Romanians had prepared a special commando unit under Lieutenant Ilie Șteflea to cross the border three hours before the start of the invasion and secure the viaduct, the 14th Romanian Infantry Regiment jumped the gun and crossed the border along with the commandos. Having lost the element of surprise, the Romanians alerted the Austro-Hungarians, who blew up the railroad viaduct. In early September, the Romanians reached Csíkszereda (Miercurea Ciuc), capital of Csik County. There, they temporarily halted their offensive and dug in on the east side of the Olt River. Further to the north, the Austro-Hungarians gave up Bélbor (Bilbor) after running out of ammunition. One brigade kept the Romanians largely east of Borszék (Borsec) while gendarmes held their own in the Kelemen (Călimani) Mountains. For the most part, however, Austro-Hungarian soldiers along the border limped towards the Transylvanian interior, accompanied by hordes of German and Hungarian refugees. For instance, over two thirds of the population of Csíkszereda left the town. However, as events would unfold, such behavior would prove exaggerated. In many places, the ethnic-Hungarian civilian population attacked small scattered groups of Romanian soldiers. A fact not merely admitted, but praised in the Hungarian press of the time, such as the newspaper Pesti Napló. In response, the Romanians nowhere took to reprisals, beyond fighting and disarming the civilians who put up active resistance. Wherever the Romanian Army advanced, proclamations were published promising safety and protection to all inhabitants, regardless of nationality. No damage was done by the Romanian armies to the property of civilians, regardless if said property belonged to ethnic-Hungarians (Székelys) or ethnic-Germans (Transylvanian Saxons).
On 8 September, the Romanians announced themselves in possession of Maroshévíz (Toplița), Csíkpálfalva (Delnița) and Gyergyószentmiklós (Gheorgheni). The Romanian North Army crossed the Olt River on 7 September. The 7th Division, after pushing aside the 19th Honvéd Brigade, continued its advance through the mountains towards Székelyudvarhely (Odorheiu Secuiesc). Having shouldered aside the 16th Honvéd Brigade at Gyergyószentmiklós, the 14th Division reached the eastern edges of the salt-mining town of Parajd (Praid) on 11 September. On 13 September, the Romanians had driven the 1st Landsturm Cavalry Brigade from the heights near Praid. By late September, the I Reserve Corps under the command of the German General Curt von Morgen, had joined the Austro-Hungarians in their fight against the Romanian North Army. The Corps succeeded in slowing the Romanians down. Von Morgen's I Reserve Corps comprised two infantry divisions: the 89th German and the 71st Austro-Hungarian. 
On 28 September, the Romanian North Army renewed its offensive. Despite stubborn Austro-Hungarian resistance, it continued to inch forward throughout the following week, advancing to within six miles of Szászrégen (Reghin) and securing the heights above Parajd by early October. In early October, the Romanian General Staff issued orders for the retreat of the North Army, due to the events unfolding in the region of Dobruja. General Arz ordered his tired forces to pursue the Romanians, but the exhausted Austro-Hungarians moved slowly, giving the Romanians an uncontested run to the border, where they settled into defensive positions prepared before the war. The pace of the pursuit is exemplified by the situation of  Székelyudvarhely. This town, capital of Udvarhely County, was taken by the Romanians on 21 September. It was retaken by the Austro-Hungarians on 8 October, however the Romanians were already gone. Having 
Northern front of the Battle of Transylvania