The Tory-Liberal Democrat Coalition made swingeing defence cuts without properly considering their strategic impact, a "devastating" report today warns.
Tens of thousands of soldiers were made redundant following the 2010 defence review.
But MPs on the Commons Defence Committee warn its findings were led by the need for cuts, not by military requirements.
Last night Conservative Defence Secretary Philip Hammond hit back saying that it was unrealistic to think that defence spending could exist in a "vacuum".
But the SNP said the report was a "devastating reality check" and attacked the 2010 decision to scrap the £3.6 billion Nimrod maritime aircraft programme.
Angus Robertson, the SNP's defence spokesman, said: "The utterly mad decision to scrap the UK's maritime patrol aircraft fleet, combined with cuts after cuts, has left the UK over-stretched, over-exposed and under-protected."
He also accused the Defence Secretary of "blissful ignorance" two weeks after a Russian warship parked off the coast of Scotland.
The report calls on ministers to produce a comprehensive national security strategy before the next defence review, due soon after the next General Election.
Mr Hammond said: "It is unrealistic to think that defence can operate in a vacuum, without having regard to the fiscal challenges the country faces.