Mashable has compiled a fairly comprehensive list of all the publicly disclosed cyber attacks on the US government (emphasis added):

Another day, another cyberattack.

Hackers accessed tax returns belonging to more than 300,000 people — more than twice officials’ initial estimate — when they breached an Internal Revenue Service program in May, stealing taxpayers’ personal information and generating nearly $50 million in fraudulent refunds, the agency said this week.

But the IRS hack is just one of more than a dozen cyberattacks on U.S. agencies in recent years, though the exact number and scope of attacks can be hard to gauge because officials are often reluctant to disclose or discuss them, let alone point fingers at suspected perpetrators. SY Lee, a spokesman for the U.S. Department of Homeland Security, told Mashable the agency did not “have a list” of cyberattacks on U.S. agencies.

“I think there’s probably some reluctance to admit the depth of the problem,” said Wayne Jackson, CEO of Sonatype. “They are way more vulnerable than they would like for us to know.”

The list turns out to be a kind of craptastic slider fiasco, but at least it auto-loads and is on a single page. Overall, it is quite informative if you have not been following these events since day one.