The kind of technical breakthrough that saves billions?
I'm not especially a fan of this approach, but the absolutist stance that you've taken seems obviously falsifiable.
None of this is justification to start a war, nor would our ethics largely permit us to proceed towards uncertain success contingent upon the sacrifice of many lives.
That said - for example the American Civil War was potentially a 'justifiable' war, costing many lives to improve lives; there is little reason to think that a technical breakthrough could not justify a war.
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NB: Besides - the GP talked about the 'technical breakthroughs outweighing the direct deaths'.
It's certainly plausible that we've saved more lives since WW1/2 than we lost in WW1/2 on account of technology developed for the war.
This has probably been studied and proven right/wrong, so it's entirely plausible to check (and to be curious about).
That's not a justification to start a war necessarily, but that's not at all GP's phrasing.