What led to a decrease in North Korea's aggressive posturing toward South Korea and the U.S.?

The death of Kim Jong-il in 2011 marked a significant turning point for North Korea. Following his passing, there was a notable shift in the country’s leadership under his son, Kim Jong-un. Initially, Kim Jong-un sought to consolidate power and establish legitimacy, which contributed to a decrease in the aggressive rhetoric and military posturing that had characterized his father's regime. This period also saw attempts to engage more diplomatically with both South Korea and the United States, including summits and discussions aimed at denuclearization.

The other choices reflect broader geopolitical dynamics or specific incidents rather than a direct cause of a decrease in aggression. While new alliances with Western countries or military drills by the U.S. may have influenced North Korea's behavior in various contexts, they do not specifically explain the changes in posturing following Kim Jong-il's death. Additionally, economic sanctions from the UN typically exacerbate tensions rather than reduce them. Thus, the leadership transition after Kim Jong-il's death was pivotal in altering North Korea's approach towards its southern neighbor and the U.S.