Excellent point. We let Assad slaughter the democratic opposition for years when we could have allowed them to prevail fairly easily against both Islamists and Assad. The West's cowardice had terrible consequences.
Please, explain how we could have "easily" ensured that moderate elements prevailed, driven out the large, well-supplied (partially by us) extremist groups, and established a stable government in the resulting vacuum. Please justify your claims with actual evidence rather than just optimistic speculation. You can't just make wild statements like that and expect people to take you at face value.