There are many examples of terrorists toppling a govt. Can either be because the govt is stupid like the Spanish who blamed the wrong terrorist group and lost an election, or be more overt, like in Palestine (Jewish terror groups in the 1930s and 1940s) or more recently the Kurds in Iraq or in South Africa with the ANC. Depending on what the goals of terrorism are, they can influence govt and the political process to remove particular political figures, or be such dumbasses in their response to terrorism, like in Spain or increasingly in the US, that voters boot them out.