The British say that there have been 14 Syrian chemical attacks since 2012 and that the last, the most horrific, killed "at least 350" Syrian civilians. The Americans count fewer attacks, but put a stunningly higher, quite precise number on the casualties: 1,429.
The French argue that only President Bashar al-Assad of Syria and the closest members of his clan can order chemical attacks; publicly, the Americans say that, at least in the August 21 attack that led US President Barack Obama to call for military action, it is unclear where the orders came from. In classified briefings they are far more specific, saying that the commander of Syria's infamous Unit 450, which controls its chemical weapons, gave the order.
In short, the differences in intelligence estimates among the United States and its closest allies are considerable but, in their view, not very significant. All come to the same bottom line: all the attacks involved sarin gas, only the Assad government had control over the chemical agents, and, whether they were premeditated or the result of "sloppiness", as one senior American official put it, the results were devastating.
As they emerge from unclassified and classified briefings, members of the US Congress say the Obama administration's case against the Assad government is convincing and leaves them with little doubt that it was responsible for the attacks.
Even those most conscious of the intelligence errors that preceded the invasion of Iraq concede that this case is different. Iraq was about assessing whether weapons existed, they say, while Syria is all about who used them, and whether a military strike would prevent - or encourage - their use again.
"More and more members of Congress are finding the evidence that Assad used chemical weapons compelling," said Adam Schiff, a member of the House of Representatives' Intelligence Committee who has been briefed on the administration's evidence.
Still, the very public way that the Americans, French, British and Israelis have felt it necessary to publish their evidence - even where it differs - underscores the huge post-Iraq sensitivities involved in justifying the need for new military involvement in the Mideast. And until the most recent gas attack in Syria, reliable assessments of the use of chemical weapons proved difficult.
The Americans say their assessment is based on "multiple streams of information, including reporting of Syrian officials planning and executing chemical weapons attacks", code words for intercepts of conversations. It also refers to "human, signals and geospatial intelligence that reveal regime activities" connected to attack preparations.
But a look at the intelligence judgment made public by the United States, Britain, France and Israel suggests that the United States was reluctant - and slow - to conclude that small-scale chemical weapon attacks began in Syria last year.
The Israelis were the first to press the case, declaring in an April 23 presentation at a security conference that they had clear evidence that Syrian forces had used chemical weapons on a small scale. But no sooner had a senior official of Israel's military intelligence unit laid out his case than US Secretary of State John Kerry, seeing the reports, called Israeli Prime Minister Benjamin Netanyahu, apparently out of concern that such a declaration would force Obama's hand.
Now the British say that in their judgment, the Syrian government "used lethal C.W. on 14 occasions from 2012", adding that "this judgment was made with the highest possible level of certainty following an exhaustive review". They added: "A clear pattern of regime use has therefore been established."
While the United States eventually came to a similar conclusion, it was with only a moderate level of confidence - meaning that some of the nation's 16 intelligence agencies disagreed. Those internal debates were not resolved until the August 21 attack, on which all the agencies agreed.
It is the French who have been the most specific. They argued in their Monday assessment that the August 21 attack involved "massive use of chemical agents" against civilian populations in several suburbs of Damascus.
During a classified briefing for about 30 lawmakers on Tuesday, US officials said that while there was no evidence that the Syrian president himself had given the orders for the most recent deadly attacks, they believe the directives came from generals close to Assad, including the commander of Unit 450.