US arms sales to Taiwan has long been a major obstacle in developing smooth bilateral relations between China and the United States, even as cross-Strait relations between Beijing and Taipei have recently become the best since 1949.

In the Aug 17, 1982 China-US Communique, the US agreed to gradually reduce its arms sales to Taiwan. However, over the years the US has continued to use the 1979 Taiwan Relations Act and the 1982 Six Assurances — treaties the Chinese government objected to — to justify the sales.

Just four months ago, the Chinese Defense Ministry urged the US to stop selling arms to Taiwan and take actions to safeguard China-US relations, following the passage of Resolution 494 by the US House of Representatives foreign affairs committee. The resolution reaffirmed the importance of the Taiwan Relations Act and the House's unwavering commitment to Taiwan. China also objected to the House's approval of the sale of four second-hand warships to Taiwan.

The Chinese protest also came just days before the arrival of a shipment of US-made Apache attack helicopters in Taiwan.

China has long called on the US to observe the One-China policy and the three China-US joint communiqués, especially the principles specified in the August 17 Communiqué, which turns 42 years old on Sunday. China has long regarded US arms sales to Taiwan as an intervention in China's internal affairs and a violation of China's sovereignty.

Despite the growing exchanges between the two militaries in China and the US in recent years, Chinese officials have described US arms sales to Taiwan, along with the US military surveillance off China's coast and the Congressional laws that restrict military exchanges with China, as three major obstacles hindering a bilateral military relationship.

According to a US Congressional Research Service report released in June, US arms transfers to Taiwan have been significant despite the absence of a defense treaty or a diplomatic relationship. Taiwan has ranked among the top recipients of US arms, the value of deliveries of US defense articles and services to Taiwan totaled $4.3 billion in the 2004-2007 period and $2.9 billion in the 2008-2011 period.

Nicholas Platt, a long-time US diplomat who went to China in 1972 on President Richard Nixon's historic trip, said the US government negotiated the 1982 document in good faith.

The document was the result of eight months of contentious negotiation to address the sensitive issue in order to avoid a collapse of the bilateral relationship.

"It's one of the fundamental documents of our relationship, and hasn't been fulfilled in every way,"Platt said.

"But it was a genuine document,"said Platt.

Platt praised both the Chinese government and Taiwan leader Ma Ying-jeou for their efforts to bring the two sides closer together in recent years. Cross-Straits exchanges have flourished. Trade between the Chinese mainland and Taiwan reached $197.3 billion in 2013, up 16.7 percent from the previous year.

Platt agreed that US arms sales to Taiwan are "obviously an issue”, but he believes the improved cross-Straits relationship has made it less of a problem now.

Richard Bush, a senior fellow and director of the Center for East Asia Policy Studies at the Brookings Institution, believes the Chinese mainland's demand that the US end arms sales to Taiwan is connected with its quest for unification.

"Deng Xiaoping's assumption, which I don't think has ever been overturned, is that arms sales obstruct unification either because they reduce the incentives for Taipei to negotiate with Beijing or they encourage proponents of Taiwan independence,"he said.

Bush believes the best way for Beijing to achieve its political objective is to make its offer more attractive to the Taiwan public, citing concerns about the growing capability of the People's Liberation Army and the one country, two systems proposed by the Chinese mainland.

Tom Watkins, an advisor to the University of Michigan Confucius Institute and economic and business entities in Detroit and the state of Michigan, described the relationship between China and the US as complicated and fragile when it comes to Taiwan.

"We have witnessed many bumps along the way in the most important bilateral relationship in the world today — between the US and China. Yet, both countries have been able to weather all storms,"he said.