On December 20th, the Romanian army crossed the Hungarian border with one armoured four infantry divisions on the front and two more in reserve. They faced the unaware defenders (two infantry divisions plus six brigades) by surprise and they broke through their lines with ease, advancing to the final objetive, the Tisza River. On December 24 Romanian forces took Nagykároly and on the 21st they took Oradea and Salonta. By the 29th, they had reached the Tisza River. To the south, Bulgarian launched two army corps against the Greek forces, but they were stop in the tracks when they met the Greek defensive line in the Nestos River.
The Hungarians, who had reinforced the Tisza line with six infantry divisions, massed heir artillery on the other side of the river and used the bridgheads of Tokai, Tiszafured, Szolnok and Szeged to lure the enemy troops into a trap where they batteries could decimated them with their concentrated fire. For three days (December 31s 1941-January 2, 1942) the Hungarian army bombarded the Romanian positions and conducted offensive reconnaissance operations. On January 5, after a fierce bombardment, Romanian infantry attacked Hungarian positions. After a days of vicious fight in Tokai, Romanian forces launched a massive onslaught against Tiszafüred. There they were halted by the defenders. The arrival of reinforcements -the 20th Infantry Division- allowed the Romanians to clear the bridgehead of Tiszafüred (January 9), only to come under heavy bombardment by the enemy batteries. There was a lull in fighting, as both sides dug in and, on January 11, the Romanians attacked in Tokai, but to no avail.
In the southern area, during a two-day battle (January 5-7), the Hungarians recovered Szentes and kept Hódmezővásárhely, which had changed hands several times between Hungarian and Romanian in those days, until it was finally lost to the Romanians (January 8). Two days later, Romanian forces occupied Szentes and Mindszent. The destruction of the Szolnok bridghead (January 10-º5) took great pains to the Romanians, but, finally, they controlled the eastern shore of the Tisza River. Then, for five days, the Romanian generals brought new reinforcements to the Tisza Line and prepared to cross the river on January 15. However, on January 13, Italy and his Balkan allies, declared war on Romania and Bulgaria and their troops crossed the border.
It's still a mistery how Mussolinni managed to conceal the build-up of his forces without being notices by neither Romanians nor Bulgarians. However, when the 3rd Italian Army entered in Romania attacking Timisoara, Mahadia and Orsova, the shock left the Romanian high command paralyzed for a few crucial hours. By the time they reacted, the forces deployed along the Serbian border had been brutally brushed aside and reduced to a pocket to the south of Timisoara. When the Italians conquered Craoiva on January 20th, they had suffered 15,000 casualties and lost 60 tanks; the Romanian army suffered less casualties (10,000) and managed to stop the enemy advance; both sides prepared for the decesive battlem that would begin on January 25, when a combined Bulgaro-Romanian army fought the Serbian-Italian forces in the battle of Stoenesti (January 25-February 26), crushing its enemies, While the Red forces were hit hard ( 75,000 casualties and lost 100 tanks and 124 aircrafts), the Bulgaro-Romanian army was crushed: 33,000 casualties and 150,000 prisoners of war. Three days later, Bucharest fell to the Italians, while Sofia surrendered on March 11. The Romanian army, with Russian reinforcements and a Bulgarian small volunter force (600-strong), would kept fighting, though March 19, when, after the battle of Buzău, the Russian forces withdrew from Romania. By then, the Romanians soldiers still fighting by their side numbered 77,000.
On March 19, 1942, Bulgaria, now led by a pro-Italian government led by Kimon Georgiev and Georgi Dmitrov, joined the RSI and declared war to Russia; King Boris III fled with the Royal Family to Spain, from where they flew, a few weeks later, to Argentina. On April 1st, Romania joined the RSI alliance with the new government of General Nicolae Radescu, who would be replaced by Constantin Parhon, who would take Romania further deep into the Socialist Revolution heralded by Mussolinni. King Carol II followed his soldiers to exile.