Joubert, Vincent. "Five Years after Estonia's Cyber Attacks: Lessons Learned for NATO?," NATO/OTAN Research Paper 76 (PDF) (May 2012).

From the report:

In April 2007 a series of cyber attacks targeted Estonian information systems and telecommunication networks ... Lasting twenty-two days, the attacks were directed at a range of servers (web, email, DNS) and routers.

The 2007 attacks did not damage much of the Estonian IT infrastructure ... However, the attacks were a true wake-up call for NATO, offering a practical demonstration that cyber attacks could now cripple an entire nation dependent on IT networks.