Lord Palmerston, a British foreign secretary and two-time prime minister under Queen Victoria, once said of Britain, "We have no permanent allies, we have no permanent enemies, we only have permanent interests." This remark seems apropos to the relationship between the United States and Vietnam following Washington's announcement on Thursday that it has partially lifted its long-time ban on the selling of lethal weapons to Hanoi.

According to US State Department spokeswoman Jennifer Psaki, US Secretary of State John Kerry told visiting Vietnamese Deputy Prime Minister and Foreign Minister Pham Binh Minh that his department "has taken steps to allow for the future transfer of maritime security-related defense articles to Vietnam".

"This policy supports Vietnam's efforts to improve its maritime domain awareness and maritime security capabilities," Psaki said at a daily news briefing.

The US move points to the closer bilateral ties between Washington and its former foe, which have seen steady improvement since the normalization of their relations in 1995, some 20 years after the end of the Vietnam War. Since 2000, both countries have shown a stronger desire to lift their relationship to a higher level. Washington approved non-lethal arms sales to Hanoi in 2006. And the two countries agreed a "comprehensive partnership" in 2013.

The trajectory of US-Vietnamese ties is a telling story of how two former arch rivals have forged a bond in the face of what they perceive to be a common interest.

Against the backdrop of the US global strategic adjustment, especially the necessity to implement its "pivot to Asia" strategy, Washington has intentionally consolidated old alliances and forged new partnerships in the region in recent years. Stronger military ties with Hanoi would help the US beef up its military presence in the region as well as make it better prepared for interfering in regional disputes, such as the territorial disputes in the South China Sea.

Hanoi is looking to the US for support in its maritime territorial dispute with China, especially since tensions between Vietnam and China escalated earlier this year amid a dispute over China's oil drilling operations in the South China Sea.

As such, it is only natural that the US' approval of selling lethal arms to Vietnam was interpreted by many people in the region as a move catering not just to the US pivot policy but also to Hanoi in its confrontation with China over the South China Sea disputes.

Hanoi needs a clear head and should understand that it is only one small piece on the US' strategic rebalancing chessboard. The growing warmth in their relations is likely to be based on fragile foundations. Due to the ideological divide between the two countries and the wounds of the war, it is difficult for Americans and Vietnamese to reverse the deep acrimony and distrust between them. For many Americans and Vietnamese, the wounds of the past are very difficult to heal.

Vietnam is never positively portrayed in US films, and there remains a strong anti-American force in the Vietnamese society which hinders each move Vietnam has taken to improve ties with its old enemy.

Besides, both need to be mindful that their strengthened military ties do not compromise each country's relationship with Beijing. After all, a head-on confrontation in the South China Sea would serve no one's interests.

The author is a senior writer of China Daily. wanghui@chinadaily.com.cn