Unlike the Cheonan incident, North Korea could not deny the shelling. Instead, it claimed to be reacting to South Korea's having first shot live artillery shells into Northern waters, claiming that it had warned the South to desist before it fired back. The background of such disputes is complex, as is the maritime geography. North Korea has never accepted the Northern Limit Line (NLL), the de facto marine border set by the UN Command (UNC) after the Korean War. Instead it claims a line of its own, extending the land-based Military Demarcation Line (MDL) westwards. This puts Yeonpyeong and other islands (including Baengnyeong, off which the Cheonan sank) in Northern waters.

In practice, the North mostly respects the NLL, but intermittently challenges it. In 1999 and 2002 the waters near Yeonpyeong saw two brief but fatal firefights between patrol boats. Another occurred in November 2009, in which some reports suggest that several North Korean sailors were killed. The sinking of the Cheonan followed in March 2010. Both Koreas hold regular military drills. In November the South and the US were engaged in their Hoguk joint exercise, held every year. The North always complains about the exercise, as it does about all such manoeuvres, claiming that they are a prelude to invasion. Separately but simultaneously, South Korean marines on Yeonpyeong were holding their regular monthly live fire drill.

The key issue is whether South Korea or the US did something out of the ordinary that might have been perceived as a genuine threat, but there is no evidence that they did. The allies fired inside South Korean-controlled waters, to the south-west of Yeonpyeong on the opposite side of the North Korean coast. The North did not claim that any of its ships were a target or in the vicinity, but merely that the shelling had hit Northern waters.

This incident, said to be the first shelling of South Korean civilians since the 1953 Armistice was enacted, was thus a dramatic and deliberate escalation of hostilities by North Korea. As with the Cheonan's sinking, there was much speculation about the North's motives, including both domestic and external goals. The former might include boosting the prestige of Kim Jong-eun, Kim Jong-il's heir-apparent, among a military that is probably sceptical of his untried youth. Significantly, both Kim Jong-il and his successor were in the vicinity on the day of the North's shelling, ostensibly to visit nearby economic facilities.