Brigadier Smyth VC OC of the 17th Indian Army Division sends Brigadier Cowen to Rangoon and he manages to convince his superior Gen Hutton that the Bilin 'River' is actually not a river but a muddy ditch and does not offer any real geographical defensive feature that Hutton's maps might otherwise suggest.
(OTL Hutton would not be budged and ordered the Green Division to hold the Bilin 'line' - where it effectively shot its bolt over several days - instead he is convinced )
Having convinced his boss he is allowed to instead collapse back to the Sitting River Bridge on the 17th Feb with the majority of the Division and its heavy equipment and MT crossing in good order before the IJA 55th Division and reinforcing IJA forces can pin it in place (and bring aircraft operating from captured airfields to interdict road travel) and using their superior numbers and experienced troops outflank the 17th Division.
The decision proves to be a sensible one as the rear guard units come under increasing air attack from the IJA bombers and fighters
It is here a far superior defensive feature that the 17th Division Makes its stand on the Eastern side of the Bridge with its supporting arms and motor transport safely on the Western side.
The Leading Japanese Regiments - 214th and 215th of the 33rd IJA Division - were forced to instead make repeated costly assaults on the Eastern End of the Bridge while the dug in Indian Army units were supported by an 18 pounder Artillery regiment and despite being WW1 vintage they were more than capable of making the difference.
After 3 days of fighting Smyth withdraws his battered 16th and 46th Brigades overnight on the 21st Feb replacing them with the fresh 1/4th Gurkha and Duke of Wellington Regiment as well as several companies from the 2nd Burma Brigade making a scratch Infantry Brigade under the command of Brigadier Punch Cowen.
This unit holds for another 2 days before the Japanese Artillery park catches up with its Division and company sized infiltration's having crossed the River North and South of the Bridge are detected after a series of skirmishes with patrols
With his troops exhausted and fearing being encircled, Smyth withdraws his scratch Brigade during the night to the Western end (undercover of a hurricane bombardment from the Divisions Artillery which fired off its reserve ammo) with Brig Cowen being the last man across before the main spans of the Bridge are dropped in a series of explosions.
The 17th Despite being battered retires in good order before dawn towards Rangoon where it comes under the somewhat improved protection of the Tomahawks of the AVG and RAF Buffaloes allowing for much greater (and safer) day light mobility by MT.
The Japanese themselves exhausted by the 5 day battle and having expended much of their ammunition spent several days reorganising before pursuing with a single Regiment supported by 2 company's of Type 95 Tanks that had been ferried across the river by the 3rd March.
The Vanguard reached Payagyi on the morning of the 4th March and in light mist the Vanguard of 4 tanks and a company of Infantry ran into an ambush by elements of the 17th Division in Brigade Strength (4 Battalions) and lose all 4 tanks and heavy losses among the infantry company
Initially thinking the opposition light the IJA Regiment made a full bloodied push later on that day only to discover that the British had rather rudely brought their own tanks - in this case 2 Squadrons of M3 Stuarts from the 7th Queens own Hussars who along with the rest of the veteren 7th Armour Brigade had arrived to reinforce the British 'Corps' in Burma.
The 1st Battle of Payagyi 4th-5th March 1942 - was possibly the first major IJA defeat at the hands of the Allies on land and the leading IJA Regiment was obliged to fall back after 2 fruitless days having suffered 1000+ Casualties and leaving behind 15 tanks and await the balance of its parent Division.
The 2nd Battle of Payagyi 10th-13th March 1942 was a much more determined affair with the Japanese using a combination of air support, artillery, tank supported infantry assaults and infiltrating light forces.
It was the latter tactic that ultimately forced the Commonwealth Brigade to retreat from its position when several light infantry columns manged to create roadblocks behind the Payagyi position effectively cutting it off
These roadblocks were felled trees roped together into a 'Giant Abatis' that was impassable to motor transport including the ubiquitous tracked universal carrier and then covered by type 96 LMGs and Knee mortars making any infantry assault attempting to clear it costly.
This had been a successful tactic previously used in Malaya and elsewhere and often forced the now cutoff unit to abandon its position as well as its heavy equipment and MT and try to 'escape on foot' with only what it could carry (Small arms and LMGs) - effectively destroying said unit as a fighting unit and often obliged to leave its wounded behind which apart from everything else was very demoralising to the unit in question.
The M3 Stuarts however were not stopped by these roadblocks and the infiltrating Japanese 'roadblock units' did not possess any weapon that could realistically engage the tanks* and both enemy roadblocks where driven off by units of the 7th Armoured Brigades and supporting infantry.
Despite the 2nd Battle being a failure as far as the IJA were concerned the subsequent decision by General Hutton the British Corps commanding officer had already made the entire result irrelevant.
With his replacement (Gen. Slim) having already told by Whitehall not to hold Rangoon and indeed to have withdrawn before the Monsoon season in April made mass movement impossible.
Hutton (who Slim did not want to replace half way through a battle) despite knowing that Rangoon was to be abandoned had been ordered to still give the appearance of holding it, which the 1st and 2nd battles of Payagyi successfully achieved.
So not wanting to lose the Brigade and in the opinion of some 'bottling it' he ordered the unit to fall back and abandon the position on the evening of the 13th March.
Much has been made of the decision - particularly on forums such as 'Alternative History dot com' where 'Rangoon is ultimately held' or 'Alexandra takes over earlier etc' and the Burma campaign is far more successful than it was - however understanding the wider picture of what was occurring elsewhere in Burma makes the decision much easier to understand.
The Japanese thrust into Burma was not just aimed at the Sitting river Bridge and then Rangoon but also further north towards China where several Chinese units (9 'Divisions') where also operating in Burma - the main strategic intention of the Japanese was to cut off the 'Burma Road' one of the major supply routes of allied materials into China - and additional Japanese forces were pushing them north and threatening to kick them out of Burma.
The more than expected resistance by the Commonwealth forces and the heavy losses suffered by the 33rd IJA Division including the majority of its armour obliged the Japanese leadership to send heavy reinforcements that had originally been intended to attack (and indeed had been) the Chinese 6th Army to join the assault on Rangoon.
So in this the British Burma Corps had achieved an important aim as it would ultimately allow the 3 Chinese army's (9 Divisions) to take up positions astride the Burma road (Mandalay to Wantang) in an orderly fashion with the British commonwealth forces ultimately holding a stable front astride the Irrawaddy river south of Yenangyoung after further hard fought battles during 1942.
Meanwhile before all of this happens the Burma Corps continued to conduct a fighting retreat with another major fight taking place at Hlegu for 3 days before the British Commonwealth forces melted away on the evening of the 19th March.
The 33rd Divisions shattered Regiments enters a 'deserted Rangoon' on the 20th and
General Sakurai Commanding officer of the 33rd Division which had conducted most of the fighting and suffered very heavy losses in the campaign to take the city wrote:
'The British have gone and with them the Golden opportunity we had of decisively defeating them has gone with them, and our prize (Rangoon) that we fought so hard for is ablaze and deserted, the port facilities destroyed and anything of use to our continued advance rendered useless'
The Burma Corps now under command of Bill Slim and comprised of the now battered but 'Veteran' 17th Indian Army Division and the reinforced Burma Division along with the 100 or so M3 tanks of the 7th Armoured Brigade defended Prome with the 200th Division (one of the better units of the NRA and including many veterans of the Battle of Kunlan pass) defending Tooungoo supported by the 55th Division NRA.
Against this position the Japanese having reinforced the 33rd and 55th division with the 18th and 56th (minus 1 regiment) Divisions along with the 213th Regiment (a detached unit of the 33rd and used to bring it up to strength - the other 2 Infantry Regiments having suffered heavy losses having formed a composite Regiment) along with 2 Tank regiments.
Initially it had been intended for the 56th Division to join the 55th in attacking the Chinese at Tooungoo during March but instead all but 1 Regiment of the 55th had been sent to reinforce the 33rd in order to ensure that Rangoon fell after the reverses the 33rd experienced and subsequently had taken over the 'chase' of teh Burma Corps while the 33rd was left to recover in Rangoon.
So the 18th joined the 55th in attacking the Burma Corps at Prome and the 56th with the 55th's 144th Inf. Rgt along with 1 of the 'new' Tank Regiments moved up towards Tooungoo where it spent several weeks trying to evict the 200th and 55th Chinese divisions during April.
OOC: Basically by being allowed to abandon the Biling 'River' line that battle is not fought and the division is able to fall back in an orderly fashion to the Sitting Bridge and dig in - OTL most of the unit had to fight its way back having 'shot their bolt' at the hard fought Billing River battle with Japanese forces blocking 2 of the Brigades while IJA aircraft strafed and bombed anything that moved and with the Bridge nearly falling on several occasions Smyth reluctantly ordered it to be blown up with the result that much of the Divisions MT and heavy equipment was lost - with 2 of its Brigades losing nearly all of its kit when it had to swim the river - the Division effectively shattered as a fighting unit with only a few of its sub units retaining their equipment and cohesion and capable of further fighting with the 'Burma Division' at the time being little better than a Militia pretending to be a division.
The IJA 55th OTL defeats the 200th NRA Division at Toungoo and then this allows the IJA to encircle and destroys the 55th NRA Division at Loikaw unhinging the entire defensive line - ITTL the 55th is obliged to reinforce the 33rd - thinking that 'The British' are going to robustly defend Rangoon and as such the 200th NRA Gets more time to prepare and is reinforced by elements of the 55th NRA when the '56th IJA Division eventually attacks in April. Again delay and heavier losses to the IJA impose significant changes to the subsequent campaign.
The delay (and losses) that the 17th instead inflicts on the IJA by instead making its stand at the Sitting Bridge and subsequently able to withdraw pretty much intact as a fighting division to inflict further defeats on the 33rd as it tries to capture Rangoon enables Slim to inherit an intact Burma Corps when he arrives in March and all of this despite Rangoon still falling creates a different situation with regards to the subsequent campaign in the rest of Burma into Late April/May when the Monsoon arrives and beyond with the campaign being conducted further East than it was OTL
Here is a map of the 'OTL campaign' Feb-April 1942 to give you an idea of what happened and where the 'places are'