You can see the list of trusted directory authorities in Tor's src/or/config.c:
https://gitweb.torproject.org/tor.git/tree/src/or/config.c#n...
There are nine of them (actually ten, but one is just for bridges), so you'd have to disrupt at least five of them to prevent them forming a majority vote on consensus together. Looks like the countries that own the IP address allocations for each dirauth are:
Austria, Germany, Germany, Holland, Holland, Sweden, US, US, US
If the above is all correct, a US<->Germany collaboration - to pick the largest set from two countries - would be one way to cause a large problem.