I remember hearing about that letter. It's certainly not good, but at the same time the context needs to be considered. I am not an expert on French politics, but there is a lot to be considered about the situation in France and around the world. The Guardian article on the subject, provided by Cheryl, is a good one and I think sums it up well.
There are elevated levels of support for the far right within the French military and so this letter should be seen in that light. It is not uncommon that elements of any state infrastructure within a country are right-leaning, by their very nature (i.e. preserving the status quo, rather than seeking change). When we see it within the military and police forces, we of course have to keep a very close eye. I don't think a coup is about to happen in France or that France is in quite the mess that the letter portrays, but of course it is concerning that the letter was written at all.
The National Front has been around in France for a long time; Jean-Marie Le Pen and his daughter, Marine Le Pen (who received ~34% of the vote in 2017...), have been in Presidential elections for many years. The growth in their support over the years is a concern and is a sign of discontent, but is not new.
The Le Pens have long targeted immigration and you're right that we have to learn from history. To use an example from the UK, rather than pointing at other nations, here is an article about parallels between how asylum seekers (refugees) are treated by the press in more recent times and how Jewish refugees were regarded by the press in the 1930s. Uncomfortable reading (e.g. see parallels at the end of the article) and a warning to us all...
https://www.theguardian.com/uk/2002/jun/08/immigration.immigrationandpublicservices
We should also be aware of the context that support for the far right has been growing in many countries. The US, Germany, France, the UK, Italy, the Netherlands, Hungary, Poland, Australia, Russia, Brazil, and many others. During times of upheaval, economic hardships, and uncertainty, people look for simplistic answers to complex issues, such as someone to blame. It can lead to dangerous situations.