It's weird enough that the Nazis adopted what was essentially the eastern four-leaf clover for their genocide party, but for a while, it really was just their party. Hitler was obsessed with the swastika, even going so far as to basically copyright it so non-government-approved entities couldn't profit off the symbol he stole fair and square. Which is why it's great that today we're free to do this to the horrifically horrendous hate-symbol:
Still, he didn't dare try to promote it to the level of the national flag. It was strictly the flag of the Nazi Party, to be flown alongside the German Imperial flag. That is until some mean ol' protesters ruined their fancy boat party.
In 1935, a "top-hat-and-tails gala" was underway on the S.S. Bremen, a German luxury liner docked at a New York pier when it was attacked by a group of merchant sailors known as the Bremen Six, who ripped down the swastika flag. The Nazis had a cow, the guys were arrested, but they were acquitted of unlawful assembly and assault by a judge who called the swastika "the black flag of piracy."
The Nazis completely melted down over that insult, and FDR actually had to kiss some ass over it, but it was hard to get the public up in arms over a party flag. It would be like if you filmed yourself eating America's stars and stripes with a fork and then slowly shitting it out vs. doing the same thing with the Democratic Party flag. One gets you shunned by society, and the other gets you a massive following on TikTok.
They wanted to make the point that the Bremen Six had attacked Germany as a whole and not just the Nazi Party, so as usual, they just bent reality to their will and passed the Reich Flag Law, making the swastika the national flag of Germany. So, lesson as always: Nazi may have been murderous monsters, but they were also enormous diaper babies.