Astonishingly, it will be his fifth cabinet job in less than a year.
The long-serving minister is also quitting as a Tory MP at the next election.
President wonders whether UK defence secretary wants "something special".
The defence secretary expressed his frustration at the NATO summit in Lithuania.
Russia And China 'Breathing A Sigh Of Relief' After Sunak Rejects Defence Spending Bid, says Tory MP
Defence secretary Ben Wallace is getting less than half what he asked for.
The defence secretary accused Russian soldiers of "rape, pillage and destruction".
But Boris Johnson has called on the UK to "break the ice" by sending fighter jets to Ukraine.
"I run a department of 224,000 people or something like that and he’s got 12 people in the office".
“There’s a lot of stories out there that I just don’t think are particularly true," the defence secretary told BBC Breakfast.
“We will plan for this year, and next year, and the year after and beyond," the defence secretary said.