The current moral argument regarding Syria is limited. The red line of chemical weapons is plotted on a wider graph of violence. Morality is relevant to the whole graph, not just the red line. For too long, innocent Syrian people have suffered greatly and lost lives at the violent hands of the regime.
If the US attacks Syria on its own, it won't be the first time in a solo operation after a British rejection. UK politicians have short memories when trumpeting the Special Relationship. Some may recall Bill Clinton's first foreign trip as President was to the UK to seek support for operations to end the bloodbath in Bosnia after four years of carnage and NATO/UN fence sitting.