They didn't find WMD's in Iraq obviously, but they were driven to investigate due to evidence. Not saying the Iraq War is justifiable on these grounds, but they had gathered evidence that led them to believe there were WMD's. Big difference between turning out to be wrong, and completely fabricating a story and arrests of Russian officials in the department of cybersecurity just to get Trump out of office. Russian interference is a very big threat, and not the first time they've tried to interfere with electoral processes, such as in France with Marine Le Pen.