What would have happened if the western forces (USA, with UK and France) have launched an offensive against East Germany during the Berlin blockade, say around 1948-1949?
If you mean, deliberately started World War III with an overall invasion absolutely nobody advocated that.
If you mean attempting to relieve Berlin with an armed convoy on the Autobahn, I had a post on that once in soc.history.what-if:
***
The 1948-9 Soviet blockade of Berlin was ultimately broken by the US airlift
(and also by something that has been rather under-emphasized: an Allied
counter-blockade that seriously hurt the economy of the Soviet zone of
Germany). However, at the onset of the blockade there was much skepticism
about whether a city as large as Berlin could be supplied by an airlift. And
even after the airlift succeeded, there were some who wished a different
approach had been tried instead:
"For all his appreciation of the airlift in action, however, [US diplomat
Robert D.] Murphy still held to his belief that, as he and [General Lucius
D.] Clay had recommended, the United States should have directly challenged
the blockade at its outset by sending an armed convoy onto the autobahn at
Helmstedt. The blockade, Murphy said some years later, was 'the one occasion
in my long career where I feel I should have resigned in public protest
against Washington's policy.' Even though he did not imagine that his
resignation would have changed the course of events, he wrote, 'I still
deeply regret that I was associated with an action which caused Soviet
leaders to downgrade United States determination and capability.' From this
point of view, the success of the airlift was irrelevant. The operation
might represent an organizational and technical triumph, but it also
represented a failure of political will." Thomas Parrish, *Berlin in the
Balance 1945-1949: The Blockade, The Airlift, The First Major Battle of the
Cold War,* p. 306.
Elsewhere in his book (p. 175), Parrish notes that in the early days of the
blockade, Clay argued "I am still convinced that a determined movement of
convoys with troop protection would reach Berlin and that such a showing
might well prevent rather than build up Soviet pressures which could lead to
war." But Clay went on to concede that if a convoy set out from the West to
Berlin, "once committed we could not withdraw."
This idea was not likely to be adopted; both the White House and the Pentagon
opposed it, as did the British. Parrish (pp. 176-7) explains the reasons for
such opposition, and the possible consequences of sending an armed convoy:
"Reacting to such ideas, [General Omar] Bradley saw in his mind's eye a
disquieting sight: a series of local political dominoes. If putting armed
convoys on the autobahn leading to Berlin should bring about a firefight
between Allied and Soviet troops and the Western forces won, the Russians
could not accept such a setback; they would have to respond with
counterattacks until they won. The same would be true in reverse if the
Soviets prevailed in the first brush--the West would escalate, but of course
only to the limited extent that the small Allied forces made possible.
Sooner or later, Bradley feared, the shooting would grow into all-out war.
Since the convoys would be moving through Soviet-controlled territory,
[Charles] Bohlen observed, the Russians could put up tank barriers, or remove
bridges and thus force the Western powers to make the first hostile move.
Robertson had the same thought. While paying tribute to the British general,
Clay apparently had no inkling that Robertson saw him as a sort of cliche
American cowboy who had to be restrained 'in his wilder moments' from sending
convoys dashing up the autobahn. Actually, Robertson had no grounds for such
fears, since he was privy to much of the cautious White House and Pentagon
thinking.
"General Ivanov, chief of staff of the Soviet occupation forces in Germany,
later observed that he and his fellow generals had not overlooked the
possibility of an American attempt to send an armored column to Berlin. But
they had not devoted much attention to devising countermeasures, Ivanov said,
because Soviet intelligence found no evidence of any actual preparations for
such an operation. Had such evidence existed, it would have had the most
serious implications, because the demarcation lines between the Eastern and
Western zones were 'holy Soviet borders.' Russians were not supposed to
cross to the West; the Allies were not supposed to cross in the other
direction. if a convoy had appeared, the Soviets might simply have erected
barriers to stop it.
"But, a Russian historian speculated, what would have happened if U.S.
personnel on such a convoy failed to maintain their self-control and opened
fire? And even before that could occur, what would junior Soviet officers do
when they saw Western tanks and armored cars rolling across their holy
border? Americans were attacking Russians, they would think--they would not
wait for the invaders to shoot but would open fire themselves. According to
Ivanov, the third possibility--'that the Soviet leadership would have yielded
to the U.S. pressure and let the convoy go free'--was ruled out in the first
discussions of the situation by the Soviet General Staff and the occupation
forces staff. The sending of a convoy would have led to unpredictable
results, most likely including armed conflict. What the Soviet leaders could
surmise but did not know definitely was that General Bradley took a similar
view. He and his colleagues on the Joint Chiefs had quite effectively summed
up the possibilities in a top-secret memorandum to the secretary of defense:
'Soviet passive resistance, such as road and bridge obstruction or
destruction, could make the armed convoy method abortive, while Soviet
interference by military action, whether simply for prevention or
deliberately as a result of war decision, would not only make the convoy
method abortive but would shift the stage from one of local friction to one
of major was involvement.'"
OK, it's not likely, but suppose Truman had taken Clay's and Murphy's advice
instead of Bradley's and Bohlen's? FWIW, many years later, Andrei Gromyko,
in answer to a question by Henry Kissinger, said "that Stalin was determined
to avoid a general war, but that he would have resisted a Western attempt to
relieve Berlin."
https://books.google.com/books?id=DHoQ5GJ2H6YC&pg=PA89