While the SolarWinds hack deserves the title of "worst cyber attack ever" (at least until the next big attack) cyber espionage has a long history. A post in Cyber Security Intelligence describes some past incidents.
The first person to discover a foreign cyber attack on U.S. military secrets was actually an astronomer, Cliff Stoll. In 1986 Stoll discovered someone was logging into his computer network without permission; he traced the login to hackers in Germany who sold access to the KGB.
The first major cyber espionage by a state intelligence agency was codenamed "Moonlight Maze" in the mid 1990s. U.S. investigators concluded the attackers were Russian, although Russia denied it. In 2008 the same group was linked to a USB stick with malware.
In 2016 two Russian intelligence teams targeted the Democratic party - foreign intelligence under cover, and military intelligence known as "Fancy Bear" leaking information to influence the election. Last year of course was the SolarWinds hack.
While Russia seems to be concentrating on military and political targets, China has been active stealing commercial secrets. The United States and United Kingdom have also been conducting cyber-espionage.
As stated in the Cyber Security Intelligence blog, as long as there are secrets online spies will be trying to steal them.