If Congress says no on the Syria issue then the President needs to follow their lead on the issue. If the President alone decided to go to Syria against Congress then he should be impeached for his actions. We need to act as a country together and not apart on big issues.
In the past we have had Presidents act unilaterally on matters of war and it hasn't gone well. Only congress can declare war. If it is voted against and a president goes to attack anyway it is a violation of the oath of office and the constitution and he should be impeached.