On 7 December 1543, the Confederation land and naval forces left Ava to start the invasion. The invaders easily overran Toungoo defenses en route to Prome, gaining complete control around Prome by late December. They launched several attacks on the city but could not take the city. By then, the Toungoo command had determined that Ava would probably not open a second front. They moved most of their troops from the Toungoo theater, and massed all their land and naval forces at Tharrawaddy , about 160 km south of Prome. In response, Hkonmaing lifted the siege, moved his army and navy to 3.2 km north of the city, and fortified their positions. The Toungoo navy led the counterattack. The larger and better equipped Toungoo war boats defeated smaller, less equipped Ava war boats. The surviving Ava war boats fled upriver, and were not heard from again. The Toungoo navy then sent 4000 troops who bypassed Prome and went on to occupy Thayet and Myede, about 60 km north of Prome, and successfully cut off from the Ava supply lines. Ava armies in the north tried to regain the strategic towns but were driven back each time. About a month later, c. late January/earlyFebruary 1544, the Confederation command decided to retreat. The retreating Confederation forces had to fight off the Toungoo armies led by Bayinnaung. They split into four smaller armies, and made their way to Salin, about 200 km north of Myede.
Question: How many months did it take for the Ava armies to be driven back?
Answer: 1