Soon after his election as president, Donald Trump attempted to crack down on speech coming out of federal agencies, especially speech on social media that might be critical of the new administration or supportive of the previous one. That attempt backfired somewhat, with federal employees going "rogue" and creating alternate social media accounts used to blast Trump policy changes.
In response, the Trump administration furthered its attempt to silence disparaging speech, issuing a federal summons to Twitter in an effort to unmask the users behind at least one rogue account. Twitter didn't blink, and countersued the Department of Homeland Security, along with several other individuals and agencies.
Less than 24 hours later, Trump's team withdrew its summons and Twitter followed suit, dropping its complaint challenging the administration's power to demand such information. You can read Twitter's filing below.