PARIS - Barack Obama famously had one, while Donald Trump and Emmanuel Macron clearly set out their own. But their "red lines" over the use of chemical weapons in the Syria conflict are now being tested.
France said Wednesday that "all indications" suggested the Syrian regime was using chlorine weapons against rebel forces.
US Ambassador to the United Nations Nikki Haley said there was "obvious evidence from dozens of victims" of breathing difficulties to corroborate chlorine attacks in rebel-held Eastern Ghouta and Idlib province.
Thus far, the Damascus regime has gone unpunished militarily for its latest suspected chemical weapons attacks in January and February.
Analysts say the regime may be seeing how far it can go.
"The attacks are part of an ongoing effort to test whether President Trump will enforce the red line on chemical weapons he put in place last year," said David Adesnik, research director of the Foundation for Defense of Democracies.
- 'Obligation to react -
Former US president Obama was the first to set out his red line in 2012, when he warned Syrian President Bashar al-Assad that the use, or even the movement, of chemical weapons would trigger US military reprisals.
But after a sarin attack the following year, which killed nearly 1,500 civilians in the Damascus suburbs according to US intelligence, Obama opted out of military reprisals and instead secured an agreement along with Russia to dismantle the Syrian chemical arsenal.
Then last April, a massive sarin attack in Khan Sheikhoun, in which 88 people died, caused an international outcry and led Trump to order strikes on a Syrian air base.
A month later after being elected French president, Macron drew a "very clear red line" on the issue, promising "retaliation and an immediate response from France" if chemical weapons were used.
Paris and Washington agreed they were ready to respond in a co-ordinated manner to any new chemical attack by the Syrian regime.
"Macron's red line has been crossed to the letter," said Bruno Tertrais, deputy director of the Foundation for Strategic Research in Paris.
"When he drew his red line, it's possible he had in mind a massive attack, like the very lethal attacks in Ghouta or Khan Sheikhoun, conducted by dedicated agents using sarin."
Instead, France in January launched a "partnership against impunity" agreed by two dozen countries to ensure that perpetrators of chemical attacks in Syria are held accountable and said it would blacklist companies and individuals it suspects of links to Syria's alleged chemical weapons programme.
For the United States, "as the president showed last April, he's willing to look at all options," the Trump administration has said, refusing to "speculate" about potential responses.
Like Paris, Washington has been focusing its attention on the political process while calling on Russia to deter the Assad regime from using chemical weapons.
"If you believe that military action would have damaging consequences on the search for a political solution, it might be legitimate not to act," said Tertrais.
"Americans do not consider chlorine to really be a chemical weapon. So they avoid the need to react," added Francois Heisbourg, president of the International Institute for Strategic Studies in London.
He warned that Macron has "taken a risk" with his red line.
"If we say and repeat that there are chemical attacks, we build an obligation to react, " he said, adding: "We are not far away from that."