The former head of the British Army has warned against military cuts in light of the Crimea crisis, which he said evokes "uncomfortable shadows of the 30s".
Lord Dannatt, who was chief of the general staff from 2006-09, said it was not time for the "US-led West to be weak in resolve and muscle".
Writing in the Daily Telegraph, he said the coalition needed to rethink its proposal to reduce the size of the regular Army by 20,000 to 82,000 by 2018, and to consider boosting numbers instead.
"Diplomacy and sanctions may be the right response for now to the Russian president, but he will look beyond those things to see where the real check on his actions might come from," Dannatt said.
Putin will only be "encouraged" by the UN and the EU's lack of resistance to force Syria's President Bashar al-Assad out of office, he added.
"Russia has long been the ally of Syria. Mr Putin will see the UN and EU as virtually powerless to impose their will on President Assad and will be further encouraged. There are uncomfortable shadows of the 30s," he added.
Dannatt warned that while economies were still struggling to recover from the downturn of 2008, there was a temptation to curb public expenditure further. However, removing resources from defence would be sending exactly the wrong message at this time.
He said: "On the contrary, there is a strong argument to recognise that the international landscape is more challenging than in 2010, and consider making a statement that greater military capability must underpin our diplomacy."
Under the 2010 strategic defence review, the government announced it wanted to withdraw all 20,000 troops in Germany, including their families, by 2020. The number of part-time soldiers will double to 30,000.