On 18 September 1970, the Syrian government became involved in Black September in Jordan when it sent a reinforced armored brigade to aid the Palestine Liberation Organization. Syrian armored units crossed the border and overran Irbid with the help of local Palestinian forces. They encountered several Jordanian Army detachments, but rebuffed them without major difficulty. Two days later, the 5th Infantry Division, heavily reinforced, was also sent into Jordan. Two armored brigades were attached to the division, bringing its tank strength up to over 300 T-55s and its manpower to over 16,000. The division entered Jordan at ar-Ramtha, destroyed a company of Jordanian Centurion tanks there, and continued directly towards Amman. Pollack says it is likely that they intended to overthrow the Jordanian monarchy itself. Despite defeating the Jordanian Army at al-Ramtha on 21 September, after fierce air attacks on 22 September, the Syrians stopped the attack and began to retreat.