Strikethrough syntax originated from the GitHub-Flavored Markdown. It's not part of the offical syntax. Regardless, here is how you will strikethrough words and sentences using Markdown.

The syntax character you are looking for on your keyboard is tildes ~.

And you put 2 of them ~~ on one end of the texts that you want to strikethrough, and another 2 ~~ on the other end.

Note: Make sure no spaces between the tildes and your targeted texts:

To strikethrough a word, let's say "war", you put 2 tildes on the left side of "war", and 2 more tildes on the right side of "war": ~~war~~.

Let's see it in action:

With Markdown editor that has real-time preview, you can see your targeted word being strikethrough:

To strikethrough a sentence, you put 2 tildes on one end of the sentence, and another 2 tildes on the other end of the sentence.

That's mouthful. Again, let's animate that:

Now let's see the strikethrough effect being applied in real-time on the entire sentence:

First, look for this quirky key ~ on your keyboard. Found it? Great. Now, place your cursor on one end of the texts that you want to strikethrough(remember, no spaces!). Now, hit the tilde key two times. You are doing very well. OK, next, carefully place your cursor on the other end of the texts(again, no spaces!). Wonderful. Finally, hit the tilde key two times again. Amazing~~