1. We are bigger than the moon.
2. We are better armed and better equipped than the moon.
3. We don't fully understand the culture or customs of the moon and it's possible inhabitants. It is therefore a possible threat and must be neutralized before it becomes a possible problem.
4. To make an example for other planets or moons who may want to fuck with us in future.
5. It may be harbouring political fugitives or terrorists, possibly even
WMD's
6. To increase interest and involvement in defence spending and war technology. Also, to distract people of earth from serious global issues at home.
7. People on the moon ain't never heard of freedum, liberdy, beer or baycun!
8.  It never shows the same face for longer than three days.
9. It's unpredictable and unstable, therefore negotiations with it will inevitably fail.
10. There's oil on the moon. 
11. The moon may be holding weapons of mass destruction. 
12. People say that some times it is bigger than usual. That is dishonest.
13. The moon is mooning us and that is clear provocation

14. We won't have a non-earther control the currents of our wave
15. Mining bitches: The moon includes aluminum, platinum, helium, magnesium, silicon, iron, olivines, pyroxenes and, yes, rare earth elements. 16.  The flag planted on the moon has been bleached white and I'll be damned if the French own the moon.
17. Pre-emptive strike on the Nazi moonbase in order to foil plans of complete world domination
18. That's no moon....it's a battle station
19. we have way too many nukes as it is, might as well use them for something
20.  the shapeshifting lizard aliens probably have a base there and they're the ones controlling the illuminati and causing famine and drought.
21. The best defense is a good offence. 

22. War drives technology, by going to war with the moon we give the planet a technological boost that will drive us forward as a species.
23. It will give all humans a common enemy. The enemy of my enemy is my friend. Thus leading to world peace. 
24. Any negative effects on the human race will be offset by survival of the fittest. Humanity will emerge as a stronger race. 
25. It will promote and improve globalization.
26. We will be able to see the stars at night better without the moon light polluting the sky.
27. werewolves will no longer ever be a threat.  
28. the moon called you fat. 
29. the moon called your mom fat. 
30. the moon does not understand or acknowledge any of your human rights. 
31. the moon is racist.

32.  the moon knows what you did last summer.