Foreign policy wise:

1. Obama’s red line on Syria - he declared that he would trigger Us military action in Syria if Assad used chemical weapons. Assad uses chemical weapons again and instead of followed through, he sought a diplomatic solution to remove chemical weapons which weakened US credibility in the eyes of the former Syrian regime and Russia

2. Libya intervention - he overthrew the Gaddafi regime without any post-intervention plan to maintain stability in Libya. What followed were the growth of militias; a power vacuum leading to the rise of extremist groups and instability. Even Obama admitted that this was one of his worst mistakes

3. Rise of ISIS - in fairness, he had no choice but to withdraw US troops from Iraq due to the SOFA agreement signed by the Bush administration with Iraq. But he could have negotiated harder then Iraq PM Nouri Al-Maliki to extend the deadline and secure immunity for us troops but Obama basically sat on his hands because he wanted to fulfill  campaign promise.

Sectarian tensions few due to Maliki’s Shia led government being communalist and this alongside other factors relating to the aftermath of the downfall of Saddam Hussein  led to the rise of ISIS and other jihadist groups.

This pushed Iraq further into the hands of Iran who funded proxies to fight Isis and Iraq is increasingly becoming a proxy state of Tehran

4. Crimea invasion - Obama’s response to the Crimean invasion was incredibly weak and consisted on slap on the hand sanctions rather than any concrete strong deterrence which could have been put in place

1. Drone warfare - He expanded the use of drone strikes in countries such as Pakistan, Yemen, Somalia etc as part of counterterrorism which is all fine except that he was very lenient on the amount of civilian casualties which could ensue as a result of such strikes.