Donald Trump keeps it simple, and likely too simple given Ukraine’s opaque and corrupt politics, when he says Joe Biden arranged the firing of a Ukrainian prosecutor looking into a company that employed his son.

You can find quotes on both sides from Ukrainians, but all have axes to grind. And why was Ukraine’s then-President Petro Poroshenko so resistant to the prosecutor’s firing that Mr. Biden had to threaten to hold back $1 billion in aid? Never explained. Some reports say the prosecutor was actually using the threat...