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The head of the British army, General Sir Nick Carter, has warned that the country's armed forces risk falling behind its potential enemies' unless there is additional investment.

There are different ways of trying to measure how a country prioritises defence - but none of these is perfect.

Let's start by looking at overall defence spending.

Last month, the House of Commons Library carried out an analysis that looked at the real-terms (ie inflation-adjusted) changes to defence spending.

It found that between 2010 and 2015 the Ministry of Defence's (MoD) budget had fallen by £8bn in real terms.

That's a cut of 18% compared with the 2009-10 budget, according to the Commons library.

Since then, spending has stabilised.

In the last financial year (April 2016 - March 2017), £35.3bn was spent on defence.

By 2020, that is forecast to increase by £1bn in real terms.

Being in a state of peace or war can influence the level of defence spending. The MoD points out that "between 2000 and 2010, defence expenditure increased quite sharply due to the conflicts in Iraq and Afghanistan".

When it comes to the numbers of military personnel, the MoD publishes monthly figures. These show the size of the British army has been steadily falling.

Britain's armed forces are now at their smallest since the Napoleonic wars.

Prof Malcolm Chalmers, of defence think tank the Royal United Services Institute (RUSI) says the UK will need a different set of capabilities - which will involve additional costs - as the nature of international threats evolve.

"The UK will need to be flexible and agile, especially when it comes to cyber- and information wars," he says.

"Thinking of fighting in terms of tank battles is not the nature of modern warfare.

"But adapting to these challenges will be capital intensive and will cost a lot of money."

How does the UK compare internationally?

A study by the International Institute for Strategic Studies (IISS) has attempted to rank the biggest defence budgets around the world.

The data reveals that the US had by far the biggest defence budget in 2016, at $605bn. Russia had the third highest defence budget ($59bn) and the UK the fifth ($53bn) - the highest of all EU countries.

That being said, the data should be treated with a degree of caution.

It's not easy to make direct international comparisons, because of factors such as fluctuating exchange rates.

In fact, the IISS caveats its own research by saying the impact of currency fluctuation could be "significant" in some countries.

There's also the question of how military expenditure should be defined.

Another way of comparing countries is to look at the proportion of national income allocated to defence. This gives an indication of how important countries see defence against other competing demands, such as health and education.

On this measure, the UK is one of the few Nato countries that meets the commitment to spend at least 2% of national income on defence. According to Nato, the UK's defence spending in 2017 was the equivalent of 2.14% of GDP.

This makes the UK the third highest-spending Nato member when it comes to defence expenditure - behind Greece (2.32%) and the US (3.58%).

However, there has been previous criticism surrounding the methodology used to calculate these numbers, such as whether war pensions count as defence spending.

This is a topic BBC Reality Check has previously examined.

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