A CANTILEVER crossing shorter than the Clyde’s Erskine Bridge spans a shallow river that marks an international border and joins two worlds.

One is modern, dynamic and an increasingly dominant force in global geopolitics and economics.

The other is paranoid, secretive and a threat to world peace.

The road and rail Sino-Korean Friendship Bridge links China and North Korea.

It offers an instant contrast for the few visitors to the hermit kingdom heading by rail from Beijing.

A bustling world of traffic jams and commerce of modern China in the border town of Dangdong transforms into silence and rural drabness just across the Yalu River and on to the Korean capital Pyongyang.

North Korea is an anachronism, a totalitarian state based on the unique concept of juche or self-reliance and the Kim dynasty’s version of Marxist-Leninism and widespread state ownership.

It sustains a dictatorial regime founded in 1948 by the late but “eternal” president Kim Il-sung and continued today by his grandson, supreme leader Kim Jong-un.

Outgoing US president Barack Obama warned incoming Donald Trump that North Korea would be his top national security priority.

And he was not wrong. Kim Jong-un’s behaviour defies predictions beyond sanctioning brutality to remain in power, most visibly when he had his uncle and number two in the regime executed for being a “counter revolutionary”.

The continued development and provocative firings of nuclear weapons into the Sea of Japan is a threat to the region and beyond.

It’s a hostile state intent on wreaking damage on the West. The US’s National Security Agency reportedly linked the rogue state to the WannaCry virus that crippled the NHS and organisations across the world.

A large team of able cyber warriors are reportedly an elite unit in the North Korean Army.

North Korea is one of the most repressive states in the world.

Information is tightly controlled. Radios come with a seal restricting reception to state channels and removing the seal is a serious offence. A flaw in recent years has been smuggled mobile phones which can receive Chinese signals in the border area, giving some citizens a glimpse of life outside and eroding the myth of a paradise state.

A hint of dissent risks years in a labour camp, with the UN estimating there are up to 120,000 political prisoners held in gulags in the harshest of conditions.

The country operates a sophisticated system where neighbours report the loyalty of neighbours, work colleagues on each other and even relatives about their families, all cross-checked and compared by the huge security establishment.

The North Korean Army is the fourth largest in the world with more than one million troops and seven million in reserve. However, while an initial assault could be devastating observers question how long its Cold War-era equipment could sustain combat.

Hence the focus on nuclear weapons development as a deterrent.

An astonishing array of artillery is arrayed along the demilitarised zone that divided North and South Korea after the 1953 Korean War armistice. It threatens the heavy concentrations of US and South Korean troops in the area and is equally concerning for the South Korean capital Seoul’s 20 million people, 35 miles away.

The only state with limited influence on North Korea is China, which shares an 870-mile border. China is Pyongyang’s biggest trading partner and the main source of energy and food.

China temporarily banned coal imports from North Korea in February for the rest of the year, perhaps losing patience with Pyongyang over its continued nuclear tests.

However, China values stability on the Korean peninsula. Hundreds of thousands of refugees pouring across the border into China if North Korea fails is a real fear for the Beijing regime, not only threatening stability in China but removing a buffer against South Korea.

Transgressions by foreigners are harshly punished, as American Otto Warmbier discovered when he was jailed for 15 years for stealing a propaganda sign.

He died this month after being sent home in a coma allegedly resulting from being tortured in prison.

Mr Warmbier’s death may be a turning point, though any military option must be constrained by three other US citizens still held in captivity.

China may hold the key and by coincidence US Secretary of State Rex Tillerson and Defence Secretary James Mattis this week host senior Chinese officials to follow up on Trump’s recent meeting with Chinese President Xi Jinping in Florida.

But, as with so much of this state where even Pyongyang’s foreign diplomats and residents readily admit they haven’t a clue about government policy, nobody knows.