Prime Minister Shinzo Abe may be finding it not so easy to live up to his pledge to make Japan a "proactive contributor to peace," as the country ends its troops' participation in a U.N. peacekeeping operation in South Sudan and looks for other missions to join.

The U.N. mission in South Sudan was the first test case for the Self-Defense Forces to play a bigger role in international peace-building efforts based on Japan's controversial security legislation, which loosened the constraints of the post-World War II pacifist Constitution.

But the SDF members are leaving South Sudan without having carried out any of the new security roles they became ready to perform just four months ago, including going to the rescue of U.N. staff and others under attack.

Tokyo has denied that the unstable situation in South Sudan, where fighting continues between government and opposition forces, was the reason for the pullout, while highlighting the results delivered through the five years of SDF activities that centered on road repair and other construction work in the capital Juba.

"The deployment of an SDF civil engineering unit has become the longest ever (among U.N. missions Japan has joined). During that time, great contributions have been made to South Sudan's nation-building," Abe told reporters on March 10 as he announced the withdrawal of the troops by the end of May.

But there has been speculation that the government opted not to take further political risks by continuing the SDF's deployment, which would have triggered a public outcry if Japanese troops had become embroiled in military actions for the first time since World War II.

Such a consequence was not necessarily unrealistic as the security legislation, among other controversial changes, has enabled SDF members to use weapons beyond just self-defense during U.N. missions, which is something they have never done in the past.

While the SDF's role overseas has gradually expanded since the 1990s, not a single bullet has been fired during such missions, in what many of the public see as symbolic of Japan's postwar pacifism.

Article 9 of the Constitution states that Japanese people "forever renounce war" and the "use of force as a means of settling international disputes."

"One death (during the South Sudan mission) would have had an impact large enough to overthrow the government. It was probably like carrying a time bomb waiting to go off," Akira Kato, an international politics professor at J. F. Oberlin University, said.

With Japan having no other SDF units taking part in active U.N. peacekeeping operations, a Defense Ministry spokesman has said the government will continue studying whether there are other "suitable" missions to send troops to.

But finding the next deployment destination without stirring controversy is likely to be a challenging task.

Japan has its so-called Five Principles, or five legal requirements governing participation in U.N. peacekeeping, to guarantee the SDF's activities will not contravene Article 9, such as the existence of a cease-fire agreement among the warring parties.

Takaki Imai, who has been visiting South Sudan to deliver emergency relief for displaced civilians as a member of a Japanese nongovernmental organization, said many U.N. peacekeeping operations are unlikely to satisfy the Five Principles, which were introduced when SDF participation in U.N. missions began in 1992.

"The nature of PKOs has changed over time...the operations were originally deployed after a peace agreement was reached and with the aim to maintain the situation, but nowadays more PKOs have been established in countries where conflicts are yet to be resolved," said Imai of the Japan International Volunteer Center.

In the case of South Sudan, the Japanese government was repeatedly questioned in parliament whether the fighting between the rival forces, which in July last year killed more than 270 people, amounted to a "combat" situation that required Japan to terminate the SDF's participation in light of the Five Principles.

Defense Minister Tomomi Inada has adamantly refused to use the term "combat" to maintain consistency with the government position that the situation in South Sudan meets the Five Principles. But daily activity logs by SDF troops, disclosed in February this year, were found to have been describing the fighting as an act of "combat."

"Contradictions appear if the government tries to send troops to U.N. missions, which in reality do not meet the Five Principles," Imai said, adding Japan should explore other forms of contribution such as sending civilians with expertise on peace issues to international organizations rather than trying to send SDF members to controversial missions.

Kato of Oberlin University, meanwhile, said the government may no longer be eager to get involved in U.N. missions not just because it is politically risky but also because its key ally, the United States, upholds an "America First" policy that focuses on its own interests and security rather than multilateralism.

The history of the expansion of the SDF's activities overseas has been closely linked to the demands of the United States amid the changing security environment, such as the end of the Cold War.

For example, Japan's first participation in U.N. peacekeeping missions was part of its efforts to move beyond "checkbook diplomacy" -- a criticism Japan faced from the United States and other countries over its support for U.S.-led forces during the 1991 Gulf War that took the form of money and not troops.

"To Japan, joining internationally cooperative actions was a way of contributing to the United States and creating a stronger alliance," Kato said, noting that in this way Tokyo sought further reassurances of the U.S. commitment to defend Japan.

"But it is becoming uncertain whether the United States, which is likely to pursue 'America First' also in military issues, will devote efforts to the defense of Japan no matter how much Japan sends SDF troops to U.N. missions to show loyalty to the United States," he added.