No wonder the state department tried to stop Marie Yovanovitch from talking. The former ambassador’s written testimony to Congress expresses clearly and in the most solemn of settings, what growing numbers American diplomats have been feeling: that their institution is being left to rot.

It is too soon to say whether her evisceration of the state department delivered behind closed doors, will mark a turning point in Donald Trump’s struggle to hold on to his office. There will without doubt be a counter-attack.

But Yovanovitch’s decision to ignore the state department gag and appear before the House committees – without having to be subpoenaed – has already cracked the wall of silence.

On Monday, the national security council’s British-born former senior director for Europe and Russia, Fiona Hill, is due to give her account. Hill, not a Trump loyalist but an expert on Russia and Vladimir Putin, resigned from her post in August. So she was in her job at the time of the infamous 25 July telephone conversation between Trump and the newly elected Ukrainian president, Volodymyr Zelenskiy.

In that call, Trump sought to persuade the comedian-turned-leader to investigate the company that had employed Hunter Biden, the son of the former vice-president and Democratic presidential contender. At the time, his aides were conditionally offering a White House visit, and Trump had put a hold on military aid the new government in Kyiv desperately needed.

On Thursday, it will be the turn of the US ambassador to the European Union, Gordon Sondland, who announced through his lawyer on Friday that he would be giving a deposition – again in defiance of state department instructions.

Sondland got his job after donating $1m to the Trump inauguration fund, but he is not a loyalist. He backed other Republican contenders for the 2016 nomination and only leapt onboard the Trump wagon when it was past the finish line.

In WhatsApp messages released by Congress, Sondland is revealed have led the effort to persuade Ukraine to launch the politically motivated investigations, bragging that he was acting on presidential authority, only to deny there was any quid pro quo when it became clear an investigation was in the pipeline.

It will be hard to stick to that story when confronted by his own words next Thursday.

Which way Sondland chooses to jump could have an important bearing on perceptions of the impeachment process among Republicans. Polls now show that half the electorate already support it.

As Yovanovitch was arriving on Capitol Hill on Friday, there were reports that more whistleblowers may be coming forward, and it seems probable that the ambassador’s words will inspire others to break ranks.

“Her quiet dignity stands in sharp contrast to the corruption of the president and many of his associates,” Nicholas Burns, former under secretary of state for political affairs, said. “Her open defiance may well encourage other career officials to come forward to tell the truth about Trump’s Ukraine policy.”

She gave a compelling account of her life: born to escapees from both Nazism and communism, an immigrant who rose to the top of US diplomacy and was appointed ambassador three times.

After more than three decades as a diplomat, under six presidents, she expressed her “deep disappointment and dismay” about what had happened to her faith that “we enjoyed a sacred trust with our government”.

“That basic understanding no longer holds true,” she said. “Today, we see the state department attacked and hollowed out from within.”

The harm would not just come from the exodus of talent, Yovanovitch said.

“The harm will come when private interests circumvent professional diplomats for their own gain, not the public good,” she went on. “The harm will come when bad actors in countries beyond Ukraine see how easy it is to use fiction and innuendo to manipulate our system. In such circumstances, the only interests that will be served are those of our strategic adversaries, like Russia, that spread chaos and attack the institutions and norms that the US helped create and which we have benefited from for the last 75 years.”

When Yovanovitch talked about private interests, it was clear what she was talking about. She had already speculated that it was private interests that were driving a quest led by Trump’s personal lawyer, Rudy Giuliani, to have her ousted from her position because of the embassy’s anti-corruption campaign.

The week’s dramatic news provided a vivid split-screen illustration of what private interests entailed. On one side a veteran diplomat walking purposefully through the halls of Congress on her way to testify; on the other, mugshots of Giuliani’s two associates, Lev Parnas and Igor Fruman, who had worked with Trump’s lawyer to look for dirt on the Bidens, and who had boasted they had helped get rid of Yovanovitch.

Booking photos for Igor Fruman and Lev Parnas. Photograph: Alexandria Sheriff/Handout/EPA

The pair are charged with funneling foreign cash into the Trump campaign and into the coffers other Republican candidates, as well as seeking to buy politicians to grant a licence for a recreational marijuana farm.

In their wake they have left a trail of fraud accusations, and failed ventures including a movie that was to be called Anatomy of an Assassin, which collapsed when investors’ money evaporated.

This was the gang that was running a parallel foreign policy in Ukraine, sidelining Yovanovitch and other career diplomats. The longer they stay in the news, the greater insight the public will have into the corruption of US diplomacy that has been under way beneath the surface for nearly three years.

Kurt Volker, the former special envoy on the Ukraine tried to work with Trump’s shadow diplomacy, reasoning that it was the most effective way to reach US policy objectives. If Giuliani and Trump were placated, US-Ukraine ties would be strengthened in the face of the threat from Russia. But when his gung-ho texts became public he simply appeared complicit.

No one who knew him doubted that he was a dedicated public servant, but very few if any who think they can handle this president come away untarnished. He resigned from his envoy position, and lost his prestigious academic post as well.

Others still in the administration are likely to learn the lessons from Yanovanitch and Volker’s stories, and decide that now is the time to make a clean break.