#1 It will likely trigger world war III. Iran has threatened to attack israel if the US invades Syria, Russia already has warships there, and China intrinsically sides with Russia.
#2 The war is based on accusation, speculation and a youtube video, with
no hard evidence such as chemical compositional analysis provided by
any independent body. To risk world war III when doubt remains is risking the human race on a guess. UN says the report on the matter will take another 3 weeks.
#3 The last time there was an accusation of sarin by the Syrian
government, the UN found out it was the Syrian rebels who had done it.  
#4 The last time there was an accusation of any weapons of mass
destruction by the western powers (US, UK, etc) using intelligence, it was wrong. No weapons of mass destruction found in Iraq.
#5 Most of the Syrian rebels are islamic extremists, notably al nusra, who are an al qaeda affiliated group. Supporting them would amount to supporting al qaeda.
#6 John Kerry was wrong in his estimates of the Syrian rebels. If he's wrong on this fundamental point - as the US secretary of state - what else is he wrong on regards to this? (Hypocrisy: John Kerry was an anti-war activist.)
#7 It's unpopular in the eyes of the American public. And would result in impeachment, possibly even rioting.
#8 The US in an economic debt of $16 trillion dollars! Each tomahawk missile costs $1.45 million!
#9 Even if the chemical weapons caches exist (which given the track
record of Iraq, Afghanistan and the previous sarin gas being
misidentified, is unlikely), blowing them up will make the chemicals
spread out into the surrounding areas potentially killing however many
#10 Blowing up Syria for using chemical weapons, especially when the UK sold the components for it to Syria and the US uses depleted uranium shells and rounds, amounts to hypocrisy.
#11 The west is not the be-all, say-all and end-all, especially when one considers how it remains unaccountable for it's own war crimes 
(hope you're happy with yourselves, military intel) and continues to
persecute people, such as Bradley Manning, Edward Snowden, Michael
Hastings, et alia, who expose the truth of their crimes.
#12 Syrian rebels have access to such chemicals as sarin, admitted to using it as such, and despite being affiliated with al nusra and thus al qaeda, aren't being targeted by the US. The same Syrian terrorists who will behead you if you don't convert.
#13 The Syrian rebels have a public track record of human rights abuses that are often visually demonstrated (Syrian rebel eats human heart, Syrian rebels use child soldiers, Syrian rebels kill civilians),
the Syrian government on the other hand only has third hand accusations
only in writing, made worse by the fact, to quote: "Rami Abdulrahman of
the Syrian Observatory for Human Rights, which is one of the
opposition-affiliated groups counting the number of those killed in the
uprising, stated that civilians who had taken up arms
during the conflict were being counted under the category of
"civilians"" (this is misleading as it makes the armed rebels look like
civilians: a civilian is a non-combatant).
#14 Such a war is untrustworthy, especially given the Saudi Arabians offered the Russians control of the global oil market
if they drop their issue on Syria (and threatened to use Chechen
terrorists to blow up the olympic winter games in Russia if they don't
comply). They even offered to fund the US invasion of Syria. The Saudi Arabians do not have the best interest of Americans at heart: only their own selfish interests.
#15 The war was preplanned by the US and UK as early as 1957,
and therefore is for selfish reasons. To quote: "Newly discovered
documents show how in 1957 Harold Macmillan and President Dwight
Eisenhower approved a CIA-MI6 plan to stage fake border incidents as an excuse for an invasion by Syria's pro-western neighbours,
and then to "eliminate" the most influential triumvirate in Damascus.".
To quote further: "a special effort should be made to eliminate certain
key individuals. Their removal should be accomplished early in the
course of the uprising and intervention and in the light of
circumstances existing at the time".
#16 The previous, non-congress approved (congress was excused baselessly) war with Libya, resulted in Libya still being in disarray. Something they're keen to keep quiet on. As well as a few posters here.
#17 It's based largely on 'rar rar' emotional rhetoric, and not on evidence, facts, or reasoning (the fact it's being rushed through before anyone has a chance to think should clue you in). The emotional
rhetoric from a child about Iraq about babies being killed with
bayonets turned out to be a lie as a PR stunt by a PR firm. Silence
on this matter is deafening. We don't need another Nayirah PR incident.
Or another WMD incident either (funny that: both fabrications).
#18 There's clearly vested interests in invading Syria, namely, oil (just two points below Libya in ranking), thus claims shouldn't be taken at face value.
#19 Every war by the US has so far resulted in disaster: Korea, Vietnam,
Afghanistan (1970s-1980s), US support for Iran-Iraq war (giving Iraq
sarin), Gulf war (Gulf war syndrome, anyone?), Bosnia/Serbia, now Iraq,
Afghanistan again (divided by sectarian tensions), US involvement in
Egypt (divided by sectarian tensions, again), Libya (divided by
sectarian tensions, again). Same screw-ups as those but with Syria,
#20 Fukushima's radioactive poisoning of the entire Pacific ocean (and possibly the world),
a much bigger issue, is largely being ignored compared to Syria, which
should raise questions of motives. Wouldn't it make more sense to deal
with Fukushima and give that more attention?
But there were only 20, lol.
Thanks to Joshua Flynn for creating this list.
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