The second Iraq war was unavoidable - the neocons have won the election in US and they were determined to test the theories they had developed over the past 30 years (elections have consequences and they absolutely had the right to make any policy they want). Which put Afganistan on the back burner.
You seem to forget Libya and before that Yugoslavia - those were wars where europe and US got involved and Russia had to stay away and cede its influence. Georgia was the opposite - Russia took action and so the west had to stay out. The same in Syria, except that in Syria the west had almost nothing to win and a lot to lose.
At the end of the day you've gotta pick your battles - picking the loose-loose ones makes you a fool.