He had articulated this idea in the exceedingly short but pointed inaugural address he had delivered, standing on the south porch of the White House on a bitterly cold morning (hatless, capeless) just before leaving for the Yalta Conference. It contained a section clearly directed not only at Stalin but at Allied decision makers wondering if he was on the level:
We have learned the simple truth, as Emerson said, that 'The only way to have a friend is to be one.' We can gain no lasting piece if we approach it with suspicion and mistrust or with fear. We can gain it only if we proceed with the understanding, and the courage which flow from this conviction.
Every action Roosevelt took in the few months left to him flowed from this idea. Churchill must have listened to the speech (and to most of the ideas FDR expressed in 1945) with a sinking heart.
Susan Butler, Roosevelt And Stalin: Portrait Of A Partnership (2015)