For five weeks, the Federal Trade Commission has questioned 30 witnesses as it has tried to prove that Meta illegally monopolized social networking through killer acquisitions of Instagram and WhatsApp.

On Thursday, the government wrapped up its side in the case — Federal Trade Commission v. Meta Platforms — setting the stage for the social media giant formerly known as Facebook to mount its defense.

During the first half of the trial in the U.S. District Court for the District of Columbia, the government grilled Mark Zuckerberg, Meta’s chief executive, alongside executives from apps including TikTok, YouTube, Pinterest and LinkedIn. F.T.C. lawyers tried to prove that Meta had overpaid for rival apps more than a decade ago in a “buy or bury” strategy.

Here are the F.T.C.’s main arguments.

That Meta Was Nervous About Rivals

The F.T.C. spent much of its time arguing that Meta had bought Instagram for $1 billion in 2012 and WhatsApp for $19 billion two years later because Mr. Zuckerberg and other executives viewed those apps as competitive threats.

Government lawyers pointed to hundreds of emails, board documents and instant messages that showed Meta executives’ near-constant paranoia about new competition.

In 2012, Mr. Zuckerberg expressed his desire in emails to lieutenants to “neutralize” Instagram, a rising competitor that had a better smartphone camera app than Facebook. Soon after, Meta executives discussed the fear that messaging apps would take on social networking features, making them competitors.