In the 23rd century, humans are at war with a technologically advanced alien species. Humans discovered a method of faster than light travel during early stages of this war when an alien capital ship was destroyed and their FTL drive reverse engineered.
Humanity, now facing the loss of the solar system and Earth if they can't turn the tide of the war, propose a new solution: blitz and raze. Basically, a few massive FTL-capable ships will be built with a single massive cannon designed to launch a projectile at a small percentage of the speed of light, which will then storm and destroy alien planets. The eventual goal is to hold an alien capital world hostage to force a peace.
Thus, what would the effect of a projectile, say a telephone-pole-sized tungsten or osmium rod, fired at a few percent of the speed of light(between 1% and 5%) at a planet be?
Would it completely destroy the planet, or merely cause an extinction level event? The damage needs to be enough to take out nuclear-hardened bases with protected production facilities for essentials such as food and ammo. An extinction-event level asteroid blotting out the sun isn't enough, ripping the planet in half would be. Would a cannon like this this work?