North Korea's missile launch this week has escalated concerns in South Korea and led to calls to deploy a United States advanced anti-missile system by July to guard against further provocations.
But this may not happen if South Korea's liberal camp - opposed to the deployment of the Terminal High Altitude Area Defence (Thaad) system - triumphs in the next presidential election, which could be held by May should the Constitutional Court decide in the next few weeks to uphold Parliament's decision to impeach President Park Geun Hye.
Much depends on Mr Moon Jae In, the main opposition Democratic Party's candidate and the race's front runner.
Mr Moon, 64, has voiced objection to Thaad on several occasions, calling it a hasty and unilateral decision by the Park administration. He has asked for the deployment plan to be reviewed, given strong opposition from China, which views it as a security threat.
Softening his stance recently, the former human rights lawyer suggested the next administration debate the issue to come to a "rational decision". The Korea Times quoted him as saying last month that he was "demanding neither the enforcement nor the withdrawal" of the plan.
Analysts said Mr Moon, hugely popular with left-leaning young voters, could be toning down his objection to appeal to conservative supporters who favour a stronger South Korea-US alliance.
Dr Lee Seong Hyon, a research fellow at the Sejong Institute think- tank, said Mr Moon is more attuned to what people want, and what they do not want - a leader like Ms Park, who makes decisions without consulting the people.
"He may have made some adjustments on his views on Thaad because this has become a contested issue. I get this funny idea that he will stick to the deployment of Thaad in the end," said Dr Lee. "What he wants to do is to give people their voice, make them feel the ownership of nationally important decisions."
Mr Moon's popularity shot up late last year after Ms Park became caught in a corruption scandal involving her close friend Choi Soon Sil.
Mr Moon, who lost to Ms Park in the 2012 presidential election, has since maintained a lead of at least 10 percentage points over rivals such as Seongnam Mayor Lee Jae Myung and South Chungcheong Governor Ahn Hee Jung, both also from the Democratic Party.
Unlike South Korea's ruling conservatives, who see North Korea as a foe, liberals like Mr Moon consider the North an estranged sibling. The Thaad deployment may run counter to their pro-North stance, but experts warn that revoking the deal could lead the US to work more closely with Japan, and leave South Korea exposed to external threats.
Dr Bong Young Shik, of Yonsei University's Institute for North Korean Studies, said Mr Moon is likely to tread carefully in handling the North and Japan, especially with regard to calls to revoke 2015's landmark comfort women deal, which remains controversial. "His hands are tied... Theoretically, he can revoke the deals, but can he weather the storm? I don't think so," said Dr Bong.
