Public outcry over Britain's foreign military policy and a reluctance from Parliament to deploy troops threatens to encourage the country's enemies, the head of the armed forces said.
Speaking on the 75th anniversary of the Battle of Britain, General Sir Nicholas Houghton said the country needs to move beyond the shadows of Iraq and Afghanistan and re-imagine the utility of the United Kingdom's armed forces in the current global context.
He was speaking at Chatham House in London, giving a personal view on the current Strategic Defence and Security Review.
And he said the military was experiencing "ever greater constraints on our freedom to use force".
He said: "Some of these constraints relate to advances in the technological competence of potential enemies and their ability to generate anti-access and area denial capabilities.
"But the more worrying constraints on the use of force lay in the areas of societal support, parliamentary consent and ever greater legal challenge.
"Such constraints are particularly significant when the desire to commit to the use of force is in support of operations which some may consider discretionary to the national interest.
"And such constraints may impact on our ability to generate deterrence, which wholly depends on the perceivable credibility of our willingness to use force if necessary.
"My point here is that if a nation's assumed willingness to commit to the use of force is only in the face of national survival, then we encourage rather than deter revisionist states and their own ambitions."
His comments came almost two years after David Cameron's historic defeat in the Commons when military intervention in Syria was opposed - the first such foreign policy reverse since 1782.
It was a decision which was met with delight from anti-war protesters, a decade after a mass demonstration attended by two million people in London, showing opposition to invading Iraq.
Anti-war sentiments from some sections of the public are only likely to be ratcheted up with the election of Jeremy Corbyn - who has repeatedly opposed military intervention and who is against replacing the Trident nuclear deterrent - as Labour leader.
Gen Houghton, the chief of the General Staff, said the country needs to become more technologically advanced if it is to tackle extremist groups that pose a threat to domestic security.
His sentiments chime with those of inter-faith charities, community leaders and police, who have called on senior Muslims to help wrestle social media from the grip of extremist groups.
Referring to the group also known as Islamic State, he said: "In Syria, Iraq and increasingly in our own homelands, Da'ish's use of messaging and propaganda is more potent than its actual conventional military capability. Da'ish uses Facebook, Twitter and Instagram in 23 different languages.
"The information age, more widely, permits adversaries unconstrained by western policy, ethical and legal codes, to exploit and assault our vulnerabilities.
"But if we are going to stay ahead of the game then we need to spend more, and more wisely on innovation."
He added: "There is no longer a simple distinction between war and peace. We are in a state of permanent engagement in a global competition. To win or even survive in such a competition means that all the instruments of national power need constantly to be in play.
"In this context we do need to re-imagine the utility of the armed forces beyond the simple construct of fighting wars or preparing for the next one."