Just a note when people say things like "Get by Mars, or Drop in by pluto and make a crawl to earth."
The planets are very, very rarely in a long chain from the Sun straight out to the outer planets. Alignments of 3-4 planets is uncommon.
Mars may have thousands of guns/torpedoes and whatnot and be a fortress, but if it's on the opposite side of the Sun from Earth, it will not have
LOS to Earth.
Here's a graphic that gives you the position of the planets today:
http://www.theplanetstoday.com/
So "getting past Mars" right now would consist of simply dropping in on the same side of the solar system as Earth is right now. Bam. Past Mars.