While UDI was strategically pointless, it must be remembered that the white Rhodesian government enjoyed so much support from the UK population as to make military action against Rhodesia, impossible. Thus the Government policy that followed was for a negotiated transition to majority rule.
From a military standpoint what the Rhodesian Army's successes did was force ZANLA and ZIPRA to recognise they could only win via massive conventional military action. Lancaster House was convened to prevent the war escalating to a state that would have forced external intervention - certainly from the UK and external raids by the Rhodesians had crippled the economies of all the states harbouring and supporting ZANLA/ZIPRA.
Now the viability and even the reality of a ZANLA/ZIPRA conventional invasion is very debatable, but the Rhodesian Army ensured the the cost of a guerilla campaign was unacceptably high, and progress unacceptably slow - thus talks were the only option. What military action ensured was a negotiated peace and not a forced one - which is what the armed insurgency sought.
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