1 Martian 2: What's our motivation for invading Earth anyway? Water? Minerals? Nicer weather? Human cattle to do our bidding? Lebensraum?
2 Martian 3: No, you idiot. It's because Earth obstructs our view of Venus.
3 Martian 2: Oh, of course...
4 Martian 1: Who did you two imbeciles learn your astronomy from? Bugs Bunny?!

This is a reference to the classic 1958 Bugs Bunny cartoon Hare-Way to the Stars, in which Marvin the Martian wants to blow up Earth for this very reason.

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2015-02-05 Rerun commentary: If you're going to have aliens invade Earth, you need to give them a sensible motivation. Fortunately there are many to choose from, as Martian 2 rattles off in the opening panel of this strip.

Astronomically speaking, it is possible for Earth to block the view of Venus from Mars, but this would only ever happen for a short period of time (the order of minutes, at an educated guess), and is so rare that it might only happen once in a million years or more. I tried Googling for a detailed orbital dynamics analysis of this problem, but for some odd reason couldn't find anyone who'd done the calculations.