09:53 Sep. 12, 2016
How do you cut a deal on military co-operation with a country you see as an existential threat?
That is how US generals characterise Russia: it is the starkest version of those "gaps of trust" President Barack Obama blamed for holding up a ceasefire deal on Syria.
And that mistrust helps explain the puzzling, even bizarre twists and turns of the week that finally ended in an agreement between the rival powers.
We now know, of course, what the deal is: the promise of joint US-Russian military action against Syria's al-Qaeda affiliate, the Nusra Front (recently renamed Jabhet Fateh al-Shams) and so-called Islamic State, which is important to Moscow.
But Russia must first put all its weight behind a ceasefire in the civil war, especially an end to bombing raids against civilians by its ally, the Syrian regime. And both must stop air strikes against US backed rebels they say are working with Nusra fighters. US allied opposition groups must observe the truce and any who are fighting with Nusra Front must break ranks with it.
It is the Secretary of State John Kerry who has doggedly promoted this unlikely military partnership as the only way to stem the violence of Syria's conflict.
Negotiations with his Russian counterpart, Sergei Lavrov, had been what administration officials called "complicated and difficult" since July, taking one step forward, then two steps back.