The crisis in Crimea shows that the UK must maintain both nuclear weapons and armed forces capable of providing a "credible deterrent", MPs have said.
The Commons Defence Committee said it was "concerned" former US Defence Secretary Robert Gates recently warned cuts would leave the UK without the "ability to be a full partner".
The UK should consider more the effect on allies of military cuts, MPs said.
No 10 says the UK is still "a first-class player in terms of defence".
The Army is undergoing a 20% reduction in regular troop numbers, from 102,000 in 2010 to 82,000 in 2020. This is to be accompanied by a rise in the number of reservists, from 24,000 to 30,000.
Similarly, the Royal Navy and the RAF are both preparing to lose 5,000 regulars and gain 500 reservists over the same time period.
Unveiling his committee's latest report, entitled Deterrence in the 21st Century, chairman and Conservative MP James Arbuthnot warned that once forces were cut back they would be "very difficult to rebuild".
He said: "Deterrence must be credible to be effective. Britain has to show the capacity and the will to respond proportionately and effectively to threats at every level.
"Recent events in Ukraine illustrate the speed with which new threats, and indeed the reappearance of old threats, can manifest themselves."
In January, Mr Gates said: "With the fairly substantial reductions in defence spending in Great Britain, what we're finding is that it won't have full spectrum capabilities and the ability to be a full partner as they have been in the past."
The spectrum refers to the ability of a country's military to fight across air, sea and land.
He singled out cuts to the Royal Navy as particularly damaging and he noted that - for the first time since World War One - Britain did not have an operational aircraft carrier.
On Wednesday, Mr Arbuthnot said: "Any proposed reductions in our conventional capabilities must be considered in the light of the effect it has on our allies - and others - rather than the purely financial."
He also predicted that the world was becoming "more multi-polar and less stable".
Given this potential deterioration in stability, the committee welcomed the "emphasis that the government places on the importance of cyber-defence".
But it warned: "We are concerned that the difficulty in identifying actors in a cyber-attack makes the ability to deter that much harder as hostile parties may feel more confident that they can mount an attack with impunity."
There were "question marks over the proportionality and legality of a response to a cyber-attack", it added.
"We call on the [Ministry of Defence] to set out in more detail in the 2015 Defence and Security Review the government's thinking on how it can deter cyber-attack from both state and non-state actors and what messaging it can employ to make it clear that an attack on vital national assets will elicit an appropriate and determined response."
Mr Arbuthnot said: "We shall increasingly need a more complex security strategy."
This should include nuclear weapons, which have "an important place in the defence philosophy of the UK", the committee agreed.
But Mr Arbuthnot added: "Strong conventional forces provide the UK with a contingency against the unexpected threats that may emerge. In a rapidly changing global environment, we may have little warning.
"Events might require the reconstitution of conventional forces, but once cut back they will be very difficult to rebuild."
Labour's shadow defence secretary Vernon Coaker said: "The report is right to raise important concerns around the impact of government defence policy on our allies' perceptions of UK capabilities, as well as echoing the serious questions around cyber-defence that Labour has asked.
"As remarked on recently by the Ministry of Defence's permanent secretary, it is clear that this government undertook their defence review with insufficient evidence.
"In contrast, we are clear that a future Labour Strategic Defence and Security Review will be both strategically ambitious and fiscally realistic."