Written by Patrick Worrall who writes the Channel 4 News FactCheck blog.
The claim
"We have British citizens going over to fight in the Israeli army. Yesterday we know they are taking part in the collective punishment of a civilian population. That's a crime."
Farooq Siddiqui, 3 July 2014
An ex-adviser to the government on tackling extremism in Britain's Muslim communities raised an interesting point on Channel 4 News in relation to Brits who fight in conflicts abroad.
Farooq Siddiqui, formerly of the Prevent programme, is calling for the UK to stop criminalising young Muslims who travel to Syria to fight against Bashar al-Assad.
Security service estimates suggest around 500 Britons have travelled to Syria to take part in the civil war.
Mr Siddiqui asked why the government has threatened to arrest British Muslims who return from Syria while it allows young people to fight for Israel and other countries with impunity.
"If we're talking about stopping people, Muslims, stopping them from going over to other countries and fighting, why are we not doing that as a blanket for stopping anyone that goes over abroad to fight in other countries?"
Is Mr Siddiqui right to say that young Brits are fighting for the Israel Defense Forces (IDF) while the Israelis are engaged in controversial strikes against Palestinian targets following the murder of three Jewish teenagers?
Image supplied by Channel 4 News
The analysis
Most Israeli citizens are obliged to do national service of up to two-and-a-half years in the country's military, and so a significant number of British-born Israelis or immigrants with dual nationality will inevitably join Israel's armed forces for a spell.
But you don't have to be a citizen of the Jewish state to fight for the IDF.
The Israeli military runs a programme called "mahal" which allows non-Israeli nationals of Jewish descent to join the ranks of the armed forces for an 18-month tour of duty.
According to the rules, British men under 24 or women under 21 who have one parent or grandparent who is or was Jewish are eligible.
That's Jewish (you need to prove it by getting a rabbi to sign a confirmation) not Israeli.
Overseas recruits get the same pay and conditions as Israelis and "serve always shoulder to shoulder with regular Israeli soldiers".
The numbers of volunteers from the UK are small but significant: the IDF told Channel 4 News there are "around one hundred Brits currently serving" in its ranks.
There is a even a support group for British parents of IDF soldiers called Mahal Mums.
We're not aware of anyone questioning the legality of this arrangement.
Unlike some other countries, Britain does not have an effective law prohibiting its citizens from fighting for foreign armies.
There is an obscure piece of legislation still on the statute books - the Foreign Enlistment Act 1870 - which ostensibly makes it illegal for British citizens to join the armed forces of a country fighting a state at peace with Britain.
But this proved to be embarrassingly ineffective when prosecutors attempted to stop British volunteers from fighting in the Spanish Civil War in the 1930s.
The lack of any practical ban on foreign enlistment leads to the slightly odd situation where a teenager can travel to Syria to fight for the brutal Assad regime with impunity, but if he sides with enemies of the regime he could face prosecution as a terrorist back in Britain.
This is because of the broad scope of anti-terror legislation. In the Queen's Speech last month, the government set out new laws which mean Brits who travel overseas to train for acts of terrorism against any government will be prosecuted as if their actions had taken place in the UK.
In May Mashudur Choudhury, 31, from Portsmouth, became the first Briton to be convicted of engaging in conduct in preparation for terrorist acts after attending a training camp in Syria.
Choudhury wasn't threatening the UK, he was training to fight Assad - who David Cameron also wanted to target with military action before losing a Commons vote last year.
But a supreme court judgement from last year ruled that the legal definition of terrorism can include "any or all military attacks by a non-state armed group against any or all state or inter-governmental organisation armed forces in the context of a non-international armed conflict".
So if someone fights in a civil a war against a regime the British government hates - even if they fight for a moderate faction not banned as a terror organisation - they can still be prosecuted as a terrorist.
War crimes?
Mr Siddiqui is not the only person to have accused Israel of war crimes over its recent actions in the West Bank and Gaza in the wake of the killings of three Israeli teens.
Amnesty International has echoed his use of the words "collective punishment", saying: "Justice will not be served by Israel seeking revenge by imposing collective punishment, or committing other violations of Palestinians' rights."
Collective punishment against a civilian population is banned under the fourth Geneva Convention.
But in the absence of any legal case brought against the IDF, the suggestion that Israeli air strikes, arrests, shootings and demolition of buildings constitute "collective punishment" of the Palestinians remains an unproven allegation.
How likely is it that Britons have been directly involved in clashes with Palestinians in recent days? Not surprisingly, we don't have information on the movements of individual IDF soldiers.
But there is no reason why they would not be involved. The rules of mahal state that overseas recruits are liable to be picked for the same frontline combat units as Israeli conscripts, including infantry, tanks and special forces.
The verdict
It was news to FactCheck, but there are around 100 British nationals serving with the IDF as we speak, apparently with no legal difficulties.
But a Brit who trains or fights with any anti-Assad rebel group runs the risk of being jailed as a terrorist.
If we are worried about young British Muslims heading off to the Middle East to receive military training, should we be equally worried about Jews?
That depends on whether Mr Siddiqui is justified in comparing the experience of serving in a professional army overseas to fighting alongside Islamist militant groups in Syria. Of course this is a politically-charged and highly debatable point.
He insisted in the interview: "It is a fighting force, whether you want to say it's disciplined or it's a militia. The effect on the individual, the effect on the combatant is still the same."