Final answer:
Western nations were hesitant to support Syrian rebel militias due to the involvement of extremist groups like ISIS, geopolitical risks including Russian support for Assad, adverse Western public opinion on military intervention, and concerns over adherence to international law.
Step-by-step explanation:
Despite being opposed to President Bashar al-Assad, Western nations were reluctant to provide support to the rebel militias in Syria for several reasons. One major concern was the presence of extremist groups like ISIS within the rebel ranks, which could potentially gain power if Assad was overthrown. Additionally, the geopolitical complexity, including the involvement of Russia in support of the Assad regime, raised the risks of further destabilizing the region and possibly escalating into a wider conflict.
Moreover, Western public opinion was largely against military intervention, as evidenced in 2013 when a proposed airstrike by President Obama faced significant domestic opposition. This reluctance was also influenced by the controversial outcomes in other Middle Eastern interventions, strengthening the aversion to becoming involved in Syria's complex civil war.
Finally, there were international law considerations, as the Assad regime's use of chemical weapons posed a direct violation of the Geneva Convention. Western nations were wary of supporting groups that might not adhere to international norms, thus further complicating the prospect of intervening in the conflict.