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With the confirmation that the MOD has received a request for support on people crossing The Channel, what is MACA, you might ask. How does it work? Educational thread alert! (This is a comment on the general principles, not this specific request) https://twitter.com/DefenceHQ/status/1292051554711810053?s=20
Military Aid to the Civil Authorities (MACA) is, broadly, activity conducted by the MOD that a) wouldn't be part of its normal daily work and b) takes place in the UK (or Crown dependencies).
It is separate from 'normal' Defence tasks like defending UK airspace against threats, exercising and training or patrolling UK waters to look out for other navies. It requires a separate category because otherwise the military running around the UK looks a lot like a coup!*
*Insert insider joke here about how Army regiments that aren't The Rifles might suggest there's already been a coup of sorts...
MACA reflects the fact that the military has a lot of capabilities, especially for moving things around or building things, which can be put into action quickly in an emergency or crisis. MACA is when the civil authorities - other departments or local authorities - request this.
Examples include using helicopters to shore up Whaley Bridge Dam, https://www.bbc.co.uk/news/uk-england-derbyshire-49201467
Helping out during flooding or other natural problems. https://www.bbc.co.uk/news/uk-51512824
And most recently setting up the Covid Support Force to help with testing. https://www.gov.uk/guidance/covid-support-force-the-mods-contribution-to-the-coronavirus-response
It also provides some capabilities that the military has that don't really exist elsewhere and wouldn't be worth civilian services duplicating; the best example probably being bomb disposal (explosive ordnance disposal - EOD - in MOD-speak). See @11_EOD_Regiment
There are several requirements in place as guidelines for MACA; mostly they boil down to exhausting other options first, though in an emergency this can be waived. MACA must be approved by a Defence minister.
How does this work? In a big crisis, COBR would probably be convened, and a department would be formally made the 'lead government department' (probably not the MOD). Or in a smaller or more contained situation an individual department or local authority would identify a need.
In any case, a formal request needs to make its way to the MOD, and specifically its 'brain', Security Policy and Operations, which advises ministers on the purpose and direction of Defence. This triggers a process in which the MOD looks at the problem and what it has available.
Sec Pol and Ops will talk to the Single Service commands, the headquarters who know what they have on a day-to-day basis, who can explain what is possible and - importantly - the cost, both financially and in terms of opportunity cost.
This is because, as mentioned, this is activity the MOD wouldn't be doing under normal daily circumstances, so it is neither budgeted nor resourced. The circumstances affect the extent to which the requesting department therefore covers the costs.
They can be asked to pay full costs (everything) or marginal costs (expenditure incurred specifically for the MACA, so things beyond wages). In exceptional circumstances, such as threat to life, they may not have to pay (zero costs).
Military activity can be expensive, so it's a significant factor in deciding whether or not to implement a MACA request. The opportunity cost might be too great for Defence (eg the relevant forces may be able to deploy) and the financial costs can make departments reconsider.
This goes into a submission to the Defence minister, the other big factor being duration; again, as this is technically activity that isn't resourced, the MOD is cautious about anything which looks open-ended. The principle is to get back to civil activity as soon as possible.
Meanwhile, the necessary structures are being established to run support. The Services all have experience in setting up local liaison and there is a Standing Joint Command (UK) to run things on behalf of the MOD, in the way that the Permanent Joint Headquarters does overseas.
SJC (UK) is based at Army HQ under @UrchTyrone, given the Army's familiarity with many of these operations, but it is a joint organisation, and individual services may be running the operation on site for SJC. https://medium.com/voices-of-the-armed-forces/standing-joint-command-sjc-resilience-of-the-uk-30e1b47c3643
MACA has gained in prominence as 'resilience' has become a bigger issue in the face of climate change, or as the forces have helped out with Uk logistical difficulties. The use of the military to 'get things done' has some attraction, both for politicians and officials.
And the threat of terrorism means that there can be calls for a military response, or a military presence as reassurance. One of the largest operations the MOD plans for, Op Temperer, is basically a huge MACA response to a terrorist attack. https://www.gov.uk/government/speeches/pm-statement-following-second-cobr-meeting-on-manchester-attack-23-may-2017
MACA is therefore a big part of Defence business, and successive defence and security reviews have mentioned it. We can expect the Integrated Review to take another look at it, given 'modern deterrence' makes a big point of the need for greater resilience at home.
Whether the military is the right response tool is a different story, along with the question of whether there should be more specific resourcing and funding for MACA as it remains strangely both frequent and yet 'unplanned' (with some exceptions, such as Temperer).
The MOD explanation of MACA can be found here. https://www.gov.uk/government/publications/2015-to-2020-government-policy-military-aid-to-the-civil-authorities-for-activities-in-the-uk/2015-to-2020-government-policy-military-aid-to-the-civil-authorities-for-activities-in-the-uk
The most recent doctrine is here. https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/591639/20170207_JDP02_Resilience_web.pdf
And the ever-excellent @commonslibrary recently did an update on MACA and Covid-19. https://commonslibrary.parliament.uk/research-briefings/cbp-8074/
