From a knowledgeable reader:

Jonah – The backstory on this is that for the last few months, various Russian defense and intelligence publications and websites have been putting out allegations that the Nazis and the Poles were in cahoots during the 1930s and planning to invade the Soviet Union. See and links from this piece. So it’s very interesting that Putin has now taken the seemingly more conciliatory position. It’s all very curious. Meanwhile the US is sending a delegation to the ceremonies marking the 70th anniversary of the start of WW2 headed by William Perry, former secretary of defense, whereas most European countries are being represented by prime ministers and similarly ranking individuals. Poland is not happy with another snub. See.

Update: From the same reader:

Thanks, Jonah! It seems that when I wasn’t looking, the administration upgraded the head of the delegation from William Perry to Gen. James Jones. Couldn’t they have at least sent Joe Biden?