A key defendant in an Iranian sanction-busting case has now become its “most important witness.”Erdogan 3c Reuters Umit BektasFollowing weeks of speculation, Reza Zarrab, a Turkish-Iranian businessman, pleaded guilty on Tuesday to charges of helping Iranian entities evade U.S. sanctions through his international network of businesses, possibly with the help of Turkish banks and government officials.

Today, Zarrab is expected to testify in a…