Tension between Moscow and Washington has steadily built for months and this attack on Malaysia Airlines flight MH17 could cripple a presidency. It’s just not yet clear which one — that of Vladimir Putin or Barack Obama.

Russia’s Mr Putin is the man under most immediate strain.

His deadly meddling in Ukraine has been decried by the West for months, but has never ceased.

Anything short of now abandoning the rebels in the east of the country is unlikely to be enough.

But Mr Obama will need to exert genuine pressure to force Mr Putin’s hand.

That won’t be easy.

The American leader has spent the past few months carefully explaining the limits of US power, warning of a ‘‘high threshold’’ for any future use of force.

In a speech in May, Mr Obama specifically highlighted the conflict in Ukraine when warning ‘‘regional aggression that goes unchecked ... could draw in our military’’.

Mr Obama’s words will now be severely tested.

Whether the Malaysian airliner carrying 298 people was deliberately targeted or hit by mistake, this marks an inescapable act of aggression.

The President’s opponents will be quick to remind people about Mr Obama’s earlier hollow pledge on Syria — that a chemical attack marked a ‘‘red line’’ the regime should not cross, and yet the US did nothing when it happened.

Mr Obama’s term in the White House is winding down.

Should he now get saddled with the words ‘‘lame duck’’, those last months will be tough.

Mr Putin is just as sensitive to the perception of weakness at home.

Russia has a lot a stake.

The country’s economic boom of recent years is built on the export of natural resources, which are vulnerable to sanctions.

Mr Putin’s attempts to undermine Ukraine are both born of confidence in Russia’s wealth — to create a sphere of influence — and paranoia that Russia could be isolated by its “enemies”.

His carefully confected stature as a Russian patriot will be threatened if he is seen to now buckle to the very Western forces he has ridiculed.

An international standoff in this type of febrile climate could quickly get out of hand.