What was shared?
According to the Washington Post, Trump disclosed highly-classified details about an Islamic State plot to plant explosives on laptops with a view to blowing up commercial airliners, and named the city in Syria where an intelligence source is believed to have gathered the information.
Did Trump commit a crime?
The president has broad powers to declassify secret material, experts agree. Some argue that just by repeating classified intelligence, the president effectively declassifies it, so no crime has been committed.
"I don't think he has done anything illegal. The president generally has the power to declassify material. But you could argue he has been reckless. Just because you are invested with legal authority to do something, doesn't necessarily mean you are right to do it," said Cristina Rodriguez, Samuel Rubin Visiting Professor at Colombia Law School.
What rules has Trump broken?
Some experts argue that divulging material that is so highly classified that the US intelligence community had not even divulged it to allies, let alone an adversary power such as Russia, could constitute a violation of the presidential oath of office.
The president swore to "preserve, protect and defend" the US Constitution, and sharing highly sensitive information with Moscow -- accused of interfering with the US election last year -- could be seen as a breach of that.
Such recklessness could be construed as "high crimes and misdemeanours" some legal scholars argue.
"It could very well rise to the level of impeachable offence. You don't have to have committed a crime for impeachment, you can face impeachment for abuse of power or abuse of public trust," said Rodriguez.
Who might investigate?
Any probe, or instigation of impeachment proceedings, would have to be launched by Congress. However, since the information was divulged to top-level Russians, and Congressional Democrats are already pushing for the appointment of a special prosecutor to investigate alleged collusion between the Trump camp and Russia, that independent prosecutor -- if named -- might also be able to take this case into consideration.
David Golove, a professor of law at New York University, said it would be one thing if Trump "just blabbed it in his inimitable manner" -- which could be construed as recklessness -- and quite another if he did it as a "sign of good faith" to build relations with Moscow: "That could be considered his judgment as a commander-in-chief, even if it might seem stupid or wrong," he said.
Will Congress impeach Trump?
Republicans are by and large standing by the president, so there is little chance of impeachment proceedings being launched. Public anger may put pressure on Mitch McConnell, the Republican Senate leader, who will have to weigh that against the political interests of his party. "As long as Trump is protected by the Republicans there is really no accountability," said Golove.
What impact on intelligence gathering?
Critics fear that America's allies will be loath to share sensitive material with US intelligence agencies if they fear the information could end up in the wrong hands. The New York Times reported the material came from Israel, and Russia has intelligence ties to the Jewish state's arch-enemy Iran.
There were also fears that Russia could exploit the information it obtained to track the source itself and prevent it being used against their own activities in Syria.
"Leaving aside the legality issue, the longstanding custom is that no one is supposed to in any way compromise sources of information," said Mark J. Rozell, Dean of the Schar School of Policy and Government at George Mason University.
