lift the world to new heights or let it fall

into a valley of disrepair.

As the world's only superpower, with an unmatched network

of alliances, America is often referred to

as the world's policeman.

But with the US police force now led by Sheriff Donald Trump,

its behaviour is increasingly unpredictable.

And, partly as a result, many of the world's rougher

neighbourhoods - from the Persian Gulf to the South China

Sea - are entering an uncertain and dangerous period.

Of course, all of these regional flashpoints

have deep local roots.

But it is also true that uncertainty

about American foreign policy is adding

to the sense of instability.

Take Iran.

For a while, the Trump administration

seemed to be pursuing a clear, if risky, policy.

It withdrew the US from the Iran nuclear accords

and seemed willing to risk a military confrontation.

But in June, Mr Trump abruptly cancelled an airstrike on Iran

that was intended to punish Iran for shooting down an American

drone.

And that's left a legacy of uncertainty,

and may have encouraged the Iranians to take further risks,

seizing three oil tankers in the Strait of Hormuz

in recent weeks.

Then there is the dangerous flare-up in Kashmir.

This also took place against a backdrop of confusing signals

from the White House.

In July, Mr Trump suggested that India

had asked him to mediate with Pakistan

over Kashmir, a statement that was immediately

denied by India.

Shortly afterwards, Delhi moved to abolish

the autonomy of the province of Jammu and Kashmir.

And that move has led to a surge in tensions

with Pakistan, with America watching from the sidelines.

And then there is Hong Kong.

The government of China is hinting

heavily at military intervention there

to suppress months of demonstrations.

A traditional US administration would express support

for the aspirations of the people of Hong Kong

and urge restraint on Beijing.

But Mr Trump has instead called the protests "riots".

On other contentious issues in East Asia

where tensions are rising, US policy is also confusing.

If China ever followed through on its frequent threats

to invade Taiwan, would the US fight to defend the Taiwanese?

The Trump administration has sent very mixed signals,

warming up dramatically the Taiwanese

at the beginning of the administration,

and then backing off.

A similar confusion reigns over the South China Sea.

The US has stepped up naval patrols there

to push back against China.

But President Trump himself regularly

questions the value of America's key alliances in East Asia,

with Japan and South Korea.

And then there is North Korea.

Mr Trump clearly greatly values his friendship

with the North Korean dictator Kim Jong Un.

But North Korea's recently restarted missile tests

and has done very little to deliver on its promise

to get rid of nuclear weapons.

So how long can the Kim-Trump rapprochement last?

Finally, there's Europe.

When it comes to the long-running conflict

in Ukraine, the Trump administration

has toughened US policy by supplying

the Ukrainian government with weapons.

On the other hand, Mr Trump himself

seems to want a much closer relationship with Russia

and with President Putin.

And he sometimes questioned the value of the Nato alliance,

sending shivers down the spine of America's European allies.

So how would Mr Trump react if there was an intensification

of the conflict in Ukraine?

The fact is, nobody can be sure.

And that's also true for all those other regional conflicts.

An unpredictable America poses new dilemmas for other world

powers whose policies increasingly

are based on guesswork about how the Trump White

House may behave.

As a result, uncertainty that begins in the Oval Office

spreads around the world, creating

waves of instability and trouble spots thousands of miles

from Washington.