Okay, this one is a bit context-sensitive, but begin with this, from Wikipedia:
On 2 July 2020, Senator Tammy Duckworth, announced her intention to block Senate confirmation of over one thousand military promotions unless defense secretary Mark Esper provided written confirmation that the Trump administration would not block Vindman's promotion to colonel. Less than a week later, Vindman announced through his lawyer that he would be retiring from the U.S. military. Vindman's lawyer, David Pressman, described "a campaign of bullying, intimidation, and retaliation" by the Trump administration as the reason for his client's retirement.
I've never heard of a President getting involved in military promotions at this low of a level – getting involved at all is pretty odd – but I'm absolutely certain this is what's happening.
For context: Vindman was part of the NSC's efforts to work with newly elected Ukrainian president Volodymyr Zelensky, and then testified to Congress, confirming the testimony of Fiona Hill, his former boss, and the ambassador to Ukraine. I have absolutely no reason to suspect the testimony was anything other than truthful, and no reason to suspect it was given for any reason other than Congress asking and his duty to serve.
If you interfere with promotions at all, it opens you up to people attempting to pander to you, whether it will work or not. When you explicitly stop the career of someone who testified to true information that made you look bad, they no longer have to guess. The US military, by order of the Commander In Chief, is now set to the task of flattering him personally, and only do their duty to the American public when they might choose to.
I wrote, some time ago, about my concerns about American military readiness at large. I no longer consider this an interesting question. If the military is structured or incentivized this way, they simply will not be able to effectively engage in any conflict against a competent adversary who sees the world clearly. And when their failure is cleaned up and obfuscated to please their Commander in Chief, the conflict will intensify. I do not know how many Americans would needlessly lose their lives over this failure. But it far outstrips whatever transient political benefit anyone achieves, is deeply immoral, and letting it happen puts the republic itself at risk.