British Foreign Secretary Philip Hammond added to the fears over possible defense cuts when he repeatedly refused to rule out a possible reduction in spending when quizzed on a TV politics show on Sunday.
"I can't tell you what will be in the Conservative manifesto and I can't prejudge the outcome of the defense and security review that will take place after the next election," he said on the BBC's Andrew Marr show. "We will protect the integrity and strength of our armed forces."
Whichever of the two scenarios comes closer to the mark "the result will be a remarkably sharp reduction in the footprint of defense in UK society over a decade," Chalmers wrote in a paper titled "Mind the Gap: The MoD's Emerging Budgetary Challenge."
The exact details of departmental budgets will become clearer once a government-wide spending review is undertaken this year. That will be followed by a strategic defense and security review, which many here believe will stretch into 2016 before it sees the light of day.
The RUSI think tanker also laid out his analysis of how much it would cost the MoD to maintain the NATO commitment of spending 2 percent of gross domestic product (GDP) on defense.
"On the current MoD planning assumption of modest real growth, and with the removal of one-off 2015-16 allocations, spending for 2016-17 is due to fall to £36 billion, equivalent to 1.85 percent of GDP," he said.
Chalmers wrote that in order to meet the 2 percent commitment in 2016-17, the MoD would need an additional £3 billion.
"Further increases would be required in subsequent years in order to keep pace with GDP growth. By 2019-20, an extension of the commitment to the NATO target would require the MoD to be provided with an additional £5.9 billion in annual spending, compared with current plans," the analyst said.
Odierno joined President Barack Obama, former British Defence Secretary Liam Fox, ex-British Army Chief Gen. Peter Wall and others who have recently sought to flag up the implications of UK defense spending slipping below the NATO commitment of 2 percent of gross domestic product after next this year.
"In the past we would have a British Army division working alongside an American division. Now it might be a British brigade inside an American division, or even a British battalion inside an American brigade," Odierno said.
The two parties are resisting attempts by the media, politicians and others to get defense and the spending cuts on the agenda during the run-up to the election.
Chalmers said that while his math may be rough, he reckoned that even on the MoD's own planning assumptions of a 1 percent real annual growth in equipment procurement and flat lining non-equipment spending, the department "could face a substantial funding gap."
That could have a damaging impact on British efforts to implement a restructuring of its armed forces known as Future Force 2020 — or as some people here call it "Future Forcer 2020s."
Gen. Nick Houghton, the chief of the defense staff, warned late last year in a speech at RUSI that failure to boost spending would damage force structures here
"I am not so blunt as to boldly state that defense needs more resources, I would though remind the next government that the force structure which this government has done so much to preserve was predicted to need real terms growth in defense funding, if it was going to be realized," he told a audience of top politicians, industry leaders and senior military officers.