The decision to block Gordon D. Sondland, the United States ambassador to the European Union, from speaking with investigators for three House committees is certain to provoke an immediate conflict with potentially profound consequences for the White House and President Trump. House Democrats have repeatedly warned that if the administration tries to interfere with their investigation, it will be construed as obstruction, a charge they see as potentially worthy of impeachment.
Now they are going to subpoena Sondland: https://www.politico.com/news/2019/10/08/trumps-eu-ambassador-ordered-to-not-give-deposition-in-impeachment-probe-000278
I don't believe the State Department can stop him from responding to a subpoena. Or maybe he just resigns and does it. By the text messages we've seen, it looks like Sondland was at least a bit complicit, suggesting they no longer text about the qui pro quo aspect of the military aid. I wouldn't be surprised if his lawyer suggested allowing the State Department to prevent his testimony to create the opportunity to distance himself from them.