Most of those are political appointments. One can classify State Department staff into "political" and "career" buckets. Political ones are expected to change whenever the administration does.
One of those guys had been with the department for
35 years. It's not just a normal house-cleaning, they've gutted the diplomatic corp of the USA, who are basically the people foreign diploments have existing relationships with.
Getting rid of all the experienced people is what normally happens when you plan to stock a department / business with crooked cronies.
“As a career diplomat, I experienced many transitions and never saw anything like this dangerous purge of public servants now underway at State,” former ambassador Laura Kennedy tweeted.
...
In the past, the state department has been asked to set up early foreign contacts for an incoming administration. This time however it has been bypassed, and Trump’s immediate circle of Steve Bannon, Michael Flynn, son-in-law Jared Kushner and Reince Priebus are making their own calls. The state department was not asked for help arranging Theresa May’s visit, according to a former official.
Yeah, basically they're sacking the whole lot of those people who have foreign policy experience and replacing them with Breitbart/FOX types. This is basically far worse than what Bush did.
Countryman’s position was especially sensitive. He was acting as “undersecretary T”, the official required to sign off on US arms sales or security assistance abroad, and the person charged with negotiating, implementing and verifying international arms control agreements and international security.
Yeah, they got rid of a senior diplomat trusted by both sides since the Reagan Administration who was in charge of preventing illicit arms dealing and was expert in anti-proliferation. Basically they can replace him with a war hawk who can e.g. approve of funneling American weapons to rebels groups against e.g. Iran, in an attempt to destabilize Iran.