Geography lesson time again I fear! Without understanding the geography and the ebb and flow of the ground war, you will find it very difficult to understand the reasoning behind the air war. It is more akin to the tactical support operations in Europe than stand alone heavy bomber campaigns of the 8th & 15th AF.
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Kolkata = Calcutta (major port supplying the Allied armies & air forces in Eastern India and the Hump)
Chattogram = Chittagong (the forward supply port for the Arakan campaigns)
Myanmar = Burma
Bangladesh was part of India until Partition in 1947 when it became East Pakistan and, after another war in 1971, a separate state named Bangladesh.
No. It wasn't a case of the Spitfire Vc squadrons "being kept back in India", as you put it, because all the fighting was on the India / Burma border areas with the forward most bases still being on the Indian side. There were 2 main campaigning areas at this time.
1. the southernmost part of India south of Chittagong (what is now Bangladesh) down the Arakan coast into Burma.
2. the area in Mauripur to the north where the Japanese invaded India seeking to disrupt Allied logistics at the main railhead at Dimapur (which meant taking places like Imphal & Kohima).
The Allies were able to switch there air forces from one front to the other as needs required.
Late 1943 saw the 3 Spitfire Vc squadrons redeployed from the defence of the Calcutta area to more forward locations in the Chittagong area, ahead of the Allied planned Second Arakan Campaign that kicked off in early Jan 1944. And why not. Japanese air activity in the Calcutta area had been limited in the preceeding months to recce flights. There was a greater need for higher performance fighters to protect the planned ground campaign from Japanese air intervention. Plans were already in hand to move the 2 Spitfire Mk. VIII squadrons from the Med (81 & 152 were withdrawn from the Italian front at the beginning of Nov) and more Spitfire VIII were en route to India by sea to allow more squadrons to convert (155 got Spitfire VIII when it was withdrawn from Imphal to Alipore in Jan 1944). Then on 5 Dec 1943, after the 3 Spitfire Vc units had been moved forward, the Japanese launched a new bombing assault on the Calcutta area under the codename "Ry Ichi-go".
Then with the Japanese invasion in the northern Imphal / Kohima area in March they were moved there, again to an area of greater need as the Japanese bombing campaign on Calcutta fizzled out.
As I pointed out previously only 3 squadrons used the Mk.Vc in India / Burma. Their basing was as follows.
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Received Vc while at Alipore (Calcutta) in Sept 1943
1 Oct moved to Amarda Road (SW of Calcutta)
15 Oct Alipore
29 Nov Ramu (inland from Cox's Bazaar south of Chittagong but still in India as it then was)
25 Feb Nidania (another base in the same area but on the coast)
21 March Rumkhapalong (just north of Cox's Bazaar)
17 April Wangjing in the Imphal area.
It converted to the Spitfire VIII in March 1944. The move to the northern front was a response to the increased threat from the Japanese invasion of India in the Imphal / Kohima region.
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6 May 1943 Alipore where it converted from Hurricanes to Spitfire Vc in Oct.
1 Nov Chittagong
13 Dec Dohazari (SE of Chittagong)
25 Feb 1944 Nazir
19 March Silchar West (west of Imphal)
5 May Dergaon (north of Kohima)
23 May Palel (south of Imphal)
10 Aug Baigachi (NW of Calcutta i.e. withdrawn from the front to rest)
Conversion to Spitfire VIII took place between June & Aug 1944
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21 June 1943 Baigachi where it converted from Hurricanes to Spitfire Vc
6 Nov Amarda Road
21 Nov Baigachi
1 Dec Ramu (near Chittagong)
25 Jan 1944 Rumkhapalong (just north of Cox's Bazaar)
5 March Sapam (south of Imphal)
11 March Wangjing (in the Imphal area)
18 April Chittagong
It converted to Spitfire VIII between Jan & March 1944
So no the Spitfire Vc units were not "being kept back". Entirely the opposite. They really couldn't be much further forward at the time.