A year on from publishing a plan to make the UK's military fit for purpose, the government still hasn't said where the money's coming from.
It now admits the long-delayed defence investment plan is still weeks away from being published.
The prime minister has committed to spend 3.5% of GDP on core defence by 2035 - but is Sir Keir Starmer willing to sanction cuts elsewhere, from welfare or the NHS, to pay for it?
As the US seemingly turns its back on NATO and the threat from Russia increases, is the UK taking defence spending seriously enough?
Niall Paterson is joined by Sky's military analyst Sean Bell.
Have you got a question for This Is Why? Email the show: why@sky.uk
