Slim, he's credited with winning the twin battles of Imphal and Kohima, regarded by most military historians who know of it as the last and latest of the "turning point" victories for the Allies alongside Stalingrad, Kursk, Alamein, Tunisia, Guadalcanal and Midway, although given the nature of jungle warfare, the numbers engaged were far smaller than all of the above examples save Midway and Guadalcanal. The battle was described by two American history profs as featuring "the most savage fighting on any front in the Second World War, including the eastern front."
The problem of course, is that the CBI theatre is known as "the forgotten war" for a reason - few reporters covered this confusing mishmash - and no wonder, since Slim's XIVth Army included Britons, Americans, Indians, Chinese, Burmese and Africans. But had Imphal gone the other way, the Japanese could have prolonged the war considerably, since China - and potentially, India - would almost certainly have collapsed in early 1944.
Before Imphal/Kohima, Slim had spent a year retraining and re-engaging the shattered Burma army to build it to elite jungle fighting standards, using tactics and approaches that resembled what Patton did with Third Army in Europe. Slim stripped down supplies to third-world standards, arguing that most supplies were wasted in the jungle - an easy feat, given an almost total lack of support from home. He created a logistical net and supply net in eastern India from scratch. He forced every unit to train to combat standards so there would be no need to guard "weak" logistics, medical or rear units, and was constantly skeptical of special ops forces because "every soldier should be an elite soldier." He created history's first successful all-air supply net, thanks to the help of US air transport units that cheerfully worked with him without the kaka that troubled allied units in Europe.
He trained troops to ignore Japanese flanking attacks ("if a Japanese force pushed into our rear, we had to learn to think as though it was the japanese who were surrounded"). He also accomplished something MacArthur never bothered to do because he was an a*shole - Slim reduced casualty rates from disease to almost zero by imposing harsh discipline on officers who did not observe rules on clothing and protective gear in the jungle, and he even gave XIVth Army a distinctive jungle warfare uniform to increase morale - "to be disciplined, soldiers have to look disciplined" (I'm paraphrasing a famous quote of his). He reorganized the medical evac system to reduce the turnaround time for casualties (since he could rarely expect replacement troops from home). He pioneered the use of Allied tanks in the jungle in the CBI theatre, coining the phrase. "the more you use, the fewer you lose" to explain his counterintuitive philosophy of jungle-based armored thrusts.
But that's just training.
Slim won Imphal by making a choice few other US/UK generals had to make - he chose to retreat. XIVth Army was in the midst of an offensive into the Arawak region; the push was designed to throw the Japanese off-balance and get Allied forces used to larger jungle operations. Unbeknownst to Slim, the Japanese had planned a concurrent and massive counterattack, designed to invade India and cut off supplies to the beleagured Chinese for ever. Although Allied troops to the south (including my grandfather with the 17th Indian Division) were doing damage to the Japanese right flank, Slim realized that by pulling his southern flank back rapidly with a forced march, he could drag the Japanese to the end of their supply lines, straining their ability to sustain the strength of their offensive. To do this, Slim would have to rely on a supply model that had already failed in Russia - he would have to supply most of the XIVth entirely by air. Sensing it was worth the gamble, he ordered the retreat.
This was also risky given that the chosen points of defence - Imphal and Kohima - didn't actually have any sizable units defending them when they were selected as the rally points. But the pullback barely succeeded in getting the troops there in time, and the Japanese were stopped there and cut to shreds in a series of engagements that resembled a jungle Stalingrad. The Japanese were fought to exhaustion - made easier by the fact that their replacements were far behind the "unexpectedly rapid" japanese advance. Then, in a Zhukovesque maneuver, XIVth counterattacked, surrounding and destroying the overextended Japanese offensive.
Later, Slim built on that record with his advance into Burma, annihilating Japanese forces in front of him with a surprise corps-sized flanking maneuver through the jungle, concluding with the battle for the key city of Meiktila. This trapped the bulk of the Japanese forces in Burma to the north, away from its transport net, pushing them against monsoon-swollen rivers and exposing the road to Rangoon to a swift armored push.
All this while commanding an army that spoke about 15 languages, with virtually no logistical support from home and lousy support from India, against an army that had performed brilliantly in the jungle in 1941-43. And, without any serious errors or defeats. If you think I'm just putting a shine on the guy because my grandfather was one of the fellas wearing a 14th Army bush hat, note that Slim was promoted to Field-Marshal and Supreme Allied commander, land forces Southeast Asia in 1945, then made governor-general in Australia, and was later made Chief of the Imperial General Staff over the objections of Montgomery, despite a lack of any political connections.
R.III