China's provocative decision to station a $1bn (£600m) deep-sea oil drilling rig in disputed waters 120 miles off Vietnam – well within Hanoi's 200-mile exclusive economic zone, in clear breach of a 2011 bilateral maritime pact and in defiance of regional and international agreements – can be explained, though not justified, in several ways.

The most prosaic explanation is that China's state-owned National Offshore Oil Corporation (CNOOC), the rig's owner, is keen to develop sources of oil and gas for the country's energy-hungry economy that do not depend on exploration agreements with western oil companies.

China regards the Paracel Islands, near the drilling site, as its sovereign territory. It takes a similar view of 90% of the 1.35m sq mile (3.5m sq km) South China Sea. The fact that nobody else agrees does not seem to bother Beijing.

"Large deep-water drilling rigs are our mobile national territory," said Wang Yilin, CNOOC's chairman, in 2012.

At the other end of the spectrum, China's move can be seen as a direct rebuff to Barack Obama, who recently completed a four-country Asia "reassurance tour" designed to strengthen regional alliances as part of his administration's so-called "tilt" to Asia.

In Tokyo, the US president warned China against forcibly pressing its maritime claims, following Beijing's unilateral declaration last autumn of an air exclusion zone over Japanese-controlled islands in the East China Sea. Obama offered specific security guarantees to Tokyo and stepped up US military co-operation with the Philippines, which is embroiled in similar disputes.

Obama denied his intention was the "containment" of China. Whatever the truth, China plainly is not ready to be contained. Its response to his strictures was to send a flotilla of 80 ships to escort the oil rig. When Vietnamese vessels challenged them, they were rammed and fired upon with water cannon.

China's action may alternatively be viewed as a warning to the 10-country Association of South-East Asian Nations (Asean), with which it has repeatedly failed to agree a long-awaited, legally binding code of conduct on maritime disputes. China is also in effect ignoring the UN Convention on the Law of the Sea.

In response to Beijing's growing assertiveness, Asean member states such as Indonesia and Malaysia have been upgrading their weapons arsenals and pursuing new defensive alliances both within the bloc and with the US and Australia.

"South-east Asian states are working to enhance their so-called 'anti-access/area-denial' capabilities. Vietnam [for example] has ordered six Kilo-class submarines," said an analysis by the International Institute for Strategic Studies. Singapore and Thailand have also joined this mini-arms race.

All the same, China's scare tactics appear to be working. Last weekend's Asean summit in Burma failed to agree on language criticising China for the final communiqué. An earlier, separate statement called only for "self-restraint" in the South China Sea without mentioning China or Vietnam by name.

This disarray reflects ongoing disagreements within Asean on how to handle an overbearing, militarily superior neighbour that is also their biggest trading and investment partner.

The current China-Vietnam confrontation can also be seen as the by-product of a troubled past. That both countries are in effect one-party, Communist-run states, and that China supported Hanoi during the Vietnam war may give a misleading impression of their overall historical relationship.

In truth, there is no love lost between the two. Vietnam was repeatedly invaded and occupied by imperial China for hundreds of years. The first Vietnamese nationalists defined their cause in relation to the threat posed by Beijing. After Vietnam's reunification in 1975, strains quickly emerged and the two countries went to war briefly in 1979. China again occupied the north while the Soviet Union backed the Vietnamese.

More recently, Vietnam's lurch into Beijing-style communist-capitalism has been accompanied by deliberate attempts to mend fences with the arch-capitalists of the US. George W Bush visited in 2006, following in Bill Clinton's footsteps. Bilateral trade has grown rapidly in recent years, as has a tentative security relationship.

The US remains wary of closer ties, however, maintaining that Vietnam must first improve its human rights record. The fact that John Kerry, the US secretary of state, and Chuck Hagel, the defence secretary, are both Vietnam war veterans may also be a factor.

This caution may be discarded if China continues to menace Vietnam and others in the region. Less than one year ago Li Keqiang, the Chinese premier, signed agreements with Vietnam on trade, infrastructure and maritime security, including recommitting China to a 2011 bilateral pact to manage peaceably their differences in the South China sea.

The deals were supposedly part of a charm offensive by China's new leadership to woo Asean countries. This seems forgotten now, as old enmities and present-day ambitions create new grounds for confrontation.