Two 5,000-strong "strike brigades" that can be rapidly deployed to help the UK respond to "diverse" threats are part of a planned £12bn increase in defence spending which the prime minister is due to outline?
But how else will the extra money be spent?
Up to 10,000 troops could be deployed in the UK in the event of a Paris-style attack, David Cameron has said, as he announced £12bn extra defence spending.
The PM said ministers wanted to replace the Trident nuclear deterrent, but the estimated cost had risen and completion been put back until "the early 2030s".
The number of civilians employed by the MoD is to be reduced by almost 30% by the end of the Parliament, he added.
Dai Hudd, the Prospect union general secretary, called this "devastating".
Setting out the government's Strategic Defence and Security Review (SDSR), Mr Cameron announced plans for investment in two 5,000-strong "strike brigades", additional F-35 jets, maritime patrol aircraft and high-altitude drones.
It comes after he held talks with French President Francois Hollande following the 13 November attacks in Paris, carried out by so-called Islamic State (IS) militants, which left 130 people dead.
Belgian police have charged a man with involvement in the attacks.
Laying out the government's £178bn defence-spending plans for the next decade, the PM announced:
- The UK's four ballistic missile submarines will be replaced, costing £31bn - up £6bn on previous estimates
- Plans for nine Boeing P8 maritime patrol aircraft, replacing the scrapped Nimrod aircraft
- Two additional Typhoon squadrons and the purchase of 24 new F35 fighter aircraft by 2023
- A reduction in the number of new Type 26 anti-submarine warfare frigates being built - from 13 to eight
- More than 20 new Protector drones, more than doubling the number of Reaper aircraft they replace
- An extra £2bn on weapons and helicopters for special forces, including the SAS
- Funding for 1,900 more spies across MI5, MI6 and GCHQ and an extra £1.9bn to be spent on cyber-security
- "Strike brigades" sourced from existing Army numbers and equipped to deploy across the globe
Mr Cameron told MPs the government had "no way of knowing" what will happen over the next five years, saying: "We must expect the unexpected.
"But we can make sure that we have the versatility and the means to respond to new risks and threats to our security as they arise."
He said he would make the case to MPs for extending air strikes against IS targets from Iraq into Syria on Thursday.
However, Labour leader Jeremy Corbyn asked the prime minister how he would apply "lessons learned in Libya, Iraq, Afghanistan and elsewhere" to the escalating conflicts in Iraq and Syria.
Liberal Democrat leader Tim Farron said "only this government could create a 'rapid reaction force' and will take 10 years to react".
Meanwhile, Angus Robertson, the SNP's leader in Westminster, hit out at plans to replace Trident, labelling it a "super expensive vanity project".
The latest defence announcements follow the 2010 defence review, when spending was cut by 8% - with Harrier jump jets, the HMS Ark Royal, planned Nimrod spy planes and thousands of jobs axed.