London (CNN) Even before he was elected everyone knew Donald Trump was a bully, what they didn't know was how his bullying would affect them, nor what it would reveal about who he fears.

Trump's often mendacious torrents of stilted rhetoric have already crushed common ground at home and polarized America.

But now a little over half way through his Presidency, having shed all but the most stubborn restraining influences in his administration, he threatens to inflict the same inflamed divisions overseas.

This week his top diplomat, Secretary of State Mike Pompeo, followed his boss's undiplomatic footsteps, dissing Britain's Prime Minister Theresa May during a trip to the UK, comparing her unfavorably to her predecessor, the venerated Margaret Thatcher.

The last time his boss was in the UK he too unloaded on the PM, suggesting May had ignored his advice on Brexit and that her former Foreign Secretary MP Boris Johnson would do a better job

None of this is random of course. It amounts to systematic bullying. This time it's because Trump wants his way on China, last time he wanted to split the EU.

Many of us figured Trump would take his aggressive impulses out on his enemies, but few realized how quickly and deeply he would divide his allies.

Like any bully he doesn't pick on people he fears; the one exception in his global tirades is Russia's Vladimir Putin, whom he meets alone and is meek to in public.

Former US President Barack Obama could lift a crowd but left a fragile legacy.

Obama's fragile legacy

Following on from President Barack Obama, Trump's tactics come as a shock.

Obama could lift a crowd, raise them with the "audacity of hope," but he over reached his political abilities, leaving a fragile legacy Trump seems intent on destroying.

When Obama visited the UK just before the Brexit vote 2016 he too had a message, but unlike Pompeo and Trump he did it in consultation with PM David Cameron, not in conflict with him.

Obama wanted to keep the UK's influential voice inside the EU, he warned British voters they'd be " at the back of the queue " for a trade deal with the US if they left the EU.

Just as Trump has done with Obama's domestic legacy, he is tearing up his international legacy too, unilaterally pulling out of Obama's signature overseas accomplishment, the Joint Comprehensive Plan of Action, known more commonly as the Iran nuclear deal.

Indeed the contrast with Obama and most modern-era US Presidents couldn't be greater: allies and enemies alike are expected to stomach Trump's lack of diplomacy, live with his lies, and move on.

Trump has brought drama to the negotiations over North Korea's nukes, upended Obama-era Asia trade plans and is now sowing division in the European Union.

The image that sums up how many in Europe feel about the US President.

Chilling global trade war

To understand the impact of all this, look no further than Trump's evisceration of polite political discourse at home -- a road map to rising tensions.

At home, Trump's surrogates routinely polarize opinion, whether Attorney General William Barr pronouncing Trump clean in the Mueller inquiry, or Brett Kavanaugh's utterly divisive Supreme Court nomination hearings, every confrontation is binary.

Like the reality TV that helped propel Trump to power, we can't turn our eyes from it.

In the US two centuries of painful political and social evolution are spiraling toward an uncertain outcome, as overseas a world order that benefited the US, in terms of stability and growth, is being upended.

There are two camps. Those are fighting back and those who aren't. The outcome however is the same, raised tensions with unpredictable outcomes.

This week Trump upped his attempted bullying of China, raising tariffs barely as their trade negotiators sat down to talks in Washington DC. They fired back a warning that "China is well prepared, determined and capable of safeguarding its legitimate rights and interests."

Global markets fear a trade war and the chilling impact that would have on all of us.

Kim Jong Un and Donald Trump seem to get on well, but few tangible results have emerged from their summits.

Diplomatic punch

With North Korea, Trump picked on Kim Jong Un threatening him with "Armageddon," but Kim had an answer, he wound Trump round his little finger telling Trump they were friends. Trump fell for it, and now Kim is back to his bad old missile-firing ways, and confrontation threatens again.

Perhaps because of what it means to Obama, Iran has come in for some of Trump's most sustained bullying, from pulling out of Obama's Iran nuclear deal, to layers of sanctions most recently this Thursday hitting their iron, steel, aluminum and copper exports, served up as a US naval armada nears Iran's waters.

Iran's begun kicking back, on Wednesday threatening to stop complying with elements of the JCPOA, putting pressure on Europe to stick with the deal -- and stick up for Iran.

Europeans told Iran to comply, creating divisions where once there were none.

The fact is, Trump is demanding his allies pay attention and support him, or face secondary sanctions.

This week in London his diplomat in chief had this demarche for US allies about Iran: "I urge the UK to stand with us to rein in the regime's bloodletting and lawlessness."

And for his UK audience already worried about a post-Brexit trade with the US, there was an unmistakable enforcer's gambit: having landed a less than diplomatic punch, Pompeo offered a kiss.

"If this is about something like commerce, let's open markets together, I know that we can."

May on the ropes

So Trump is trampling over his allies, and they're struggling to deal with it.

At the NATO summit last summer Trump accused Germany's Chancellor Angela Merkel of being a "captive" of Russia, a month earlier he snubbed near neighbor Canadian Prime Minister Justin Trudeau at the G7 summit in Quebec, turning up late, leaving early and refusing to sign the communique.

Neither leader responded with like-for-like bombast, but even so distaste for his tactics growing.

Last summer Theresa May too, chose to shrug off Trump's attack on her leadership.

In Trumpian style he apologized, claimed to be misquoted. The newspaper responsible published the whole transcript, proving he was wrong.

Like any bully, Trump presses hardest when his opponent shows weakness. May even then was on the political ropes, now she's even closer to a knockout amid the Brexit chaos. This is perhaps why Trump and Pompeo feel they can pick on her again, perhaps break the UK free of the EU and other allies.

It's a playbook his administration is adopting: Pompeo and National Security Advisor John Bolton seem increasingly unfettered from restraint. They have become Trump's global enforcers, putting allies and enemies under maximum pressure to get what the boss wants.

Its uncharted territory, and while Trump might be able to keep a lid on the passions he inflames at home, slicing up global alliances without a plan of how to stitch them back together is a far riskier proposition.