In their defense of Burma, the British falsely assumed that Siam would remain neutral, meaning that if Japan did invade, they would approach from the sea. The first Japanese attacks began in January, aided by Siamese forces and the newly formed Burmese Independence Army. Progress was quickly made from the south against minimal resistance whilst the majority of Japanese forces entered Burma through Thailand. Soon Rangoon came under attack from ground troops and air raids from air bases the Japanese had captured during their initial push. Under the threat of being surrounded, British forces in the area retreated north towards Prome. British forces and equipment were left stranded when the bridge at Sittang was blown by retreating troops. Throughout the campaign Japanese tactical intelligence in the tough regional terrain forced the retreat of the allied British and Chinese soldiers even though they possessed superior numbers.