There is no exit ramp for superpowers leaving center stage, and even if there was, Trump and quite rightly all Americans have no intention of taking it, abdicating most influential nation status.

With such influence comes a power to shape the world's destiny in one's own image and to one's needs. History, even the stuff written last week, is full of generations fighting in that struggle to keep the top spot.

A glimmer of existence vs. an eternity

The aspiration for nationhood itself is powerful. At the farthest end of the spectrum, take the Catalan Republic declared last week. It glowed brightly for a few minutes in the eyes of its beholders, secessionist Catalonians, before being snuffed out by Spain's government in Madrid.

A glimmer of existence so brief it risks the ignominy of being Europe's shortest lived country, and never had a hope of being officially recognized in the global constellation of nations, never mind aspiring to world power.

The Greeks and Romans flung themselves far and wide, throwing up empires built on the backs of slaves, the remains of which dot the world today.

They gave us great skills -- the Romans, underfloor heating and aqueducts, the Greeks, philosophy, mathematics and demos (democracy) -- but both burnt out under the weight of global power.

Chinese dynasties came and went, as did the Persians. Cuneiform script, one of the world's first recognized writings, was a creation of the Sumerian empire discovered in what today is known as Iraq -- and who today has ever heard of the Sumerians?

Portugal and Spain had their days of empire and influence. Renaissance Italy bequeathed the world art of staggering beauty, as groundbreaking in its day as the first mobile phone selfie a decade or so ago. All three are now shadows of their former glory.

Britannia, too, ruled the waves for a while, grew a commonwealth to protect its global interests, but like all the other super powers that came before, arrived at an unforeseen apogee before bending back toward earth.

Perhaps one measure of the arc is the time it takes to downsize an empire back to its roots.

But like life, empires have a few certainties: taxation if the empire is to survive and death when the money is gone.

Not until Scotland, Northern Ireland and Wales all fracture off from England -- not as absurd-sounding a proposition as it was 20 years ago -- and not until pro-EU London tires of Brexit and votes to secede from England will Great Britain's greatness truly be spent.

It will leave a legacy of Magna Carta, of old castles, warm beer, stiff upper lips, laws, and bureaucracy, but most of all how it managed its decline.

The at times ugly struggles for independence across Britain's collapsing Empire serve as a bloody reminder of how perilous downsizing is. Not just in terms of the staggering numbers of lives lost, but in how contemporary and coming generations will judge you.

Architect of America's ouster?

Times of change are the most dangerous, and in China, Trump will face the architect of America's ouster from most powerful status.

How both countries handle this is an existential issue for the rest of us.

While North Korea may dominate the headlines as Trump tries to cajole Xi into tougher sanctions on the Hermit Kingdom, the unspoken narrative will be Trump's efforts to burnish his own credentials as a man of true global power, exerting his will over Xi's reticence.

It may all be a vain effort to put Kim Jong UN's nuclear genie back in the bottle, but it will keep other deeper and potentially far more dangerous issues regarding Sino-US relations out of the big headlines.

In Africa, China's massive new military hub in Djibouti casts a long shadow over the nearby US military base and sphere of regional influence, since it shows China is getting ready to back up its global interests with hard power.

In Pakistan, China's massive financing of the huge Gwadar Port project threatens the fragile balance of interests that the US relies on to bend Pakistan to its strategy to end the war in Afghanistan and bring US troops home.

In Asia generally, the tectonic plates of global change are inching forward, threatening America's unchallenged superpower status, and President Xi is the force keeping them moving.

Trump's Asian tour also stops in the Philippines, Japan and Vietnam, where he will find potential allies in pushing back against China's claims to sea territory and disputed islands -- but even together, they are unlikely to persuade China it is not in the ascendency.

Today's superpower, the United States, has by the measure of modern nations risen on a relatively steep arc: nationhood some 241 years ago, leader of the free world about 70 years back.

How long the plateau lasts, how steep the possible fall, is currently in President Trump's hands -- and how he plays that hand during his nine-day tour of Asia, beginning this weekend, may well shape the arc of America's remaining years as number one nation.