You always hoped you could be a "good" neighbor and get along with the people who lived near you. For a while, it worked. You loved your neighborhood and you seemed to click with everyone. It made your life fairly stress-free, which you enjoyed.
Then a family next door moved out and you got new neighbors. They hadn't been there a month before you had a serious problem on your hands: They built a fence on your property.
You were gone when they built it, so you couldn't stop them. You calmly, politely explained to them that they'd built it in the wrong place and that they needed to remove it, but they just laughed, said it was on their land and refused to do anything.
Writing a letter
You decided to take things a step further. If they weren't going to listen to you, you were going to write a strongly worded letter telling them off. You were going to force them to make the necessary changes and show them that this was not how people did things in your neighborhood. Who did they think they were?
Is it a good idea? Probably not. For one thing, it's an emotional response. You're angry about what they did and how they treated you. In almost all situations, you never want to act while you're angry. Do not write them an angry, insulting letter. You may end up regretting it later, depending on what you say.
Secondly, these letters don't really accomplish anything. You already talked to them and they wouldn't move the fence. Is putting it in writing going to convince them? It's probably just going to escalate the situation when they feel attacked and write an angry letter back. It could make you furious if they just rip it up, throw it away and ignore it.
But what it's not going to do is make them move that fence.
This doesn't mean you have no options. You may want to have a lawyer write the letter, which can give it a bit more clout. They won't listen to you, but will they listen to a legal professional?
You also want to consider things like having a survey carried out to establish exactly where the property line is. If you're right and the fence is on your land, then the law is on your side. There are things you can do to resolve the dispute in your favor. Don't get angry. Don't act rashly. Just find out what legal steps you need to take in Washington.