Today, Nelson Mandela would be celebrating his 100th birthday.

When he was alive, South Africans used the day to celebrate him. That tradition continues to this day, even though he's no longer with us.

On "Mandela Day," as July 18 is known, you're supposed to channel your inner-Mandela: Give time to others. Listen. Share with the less fortunate. Be kind to one another.

There is always the danger of faux jubilation and mock piety with annual commemorations -- that pausing to think and share on Mandela Day is a bit like going to church once a year at Christmas.

Obama warns against 'strongman politics' in speech after Trump news conference

There's also the danger that Mandela's legacy becomes hollowed out, simplified and that he's turned into a false deity of righteousness. After all, there's no shortage of people flogging Mandela trinkets, T-shirt or books of quotes.

However, South Africans take the day seriously. The 100th anniversary of the birth of the man who defined strength, forgiveness and reconciliation is viewed with even more solemnity than usual. So much so, the former US President Barack Obama made one of his first post-presidency public speeches in Johannesburg in honor of Mandela.

With world politics turned topsy-turvy and South Africa, specifically, ravaged by the corruption of the Zuma years, there's much on which to reflect.

The question being asked is, if he were around now: "What would Mandela do?"

Mandela was masterful in dealing with both friends and foes.

As world leaders and diplomats grapple with the contradictions of the Trump presidency, you wonder how Mandela would have worked with him and responded to the impact he's having on the international stage.

I don't doubt that he would have unleashed a charm offensive on Trump.

Mandela had no problem with flattering people. He wooed CEOs and the wealthy and then, with a smile on his face, fleeced them for cash for rural schools or some other social investment. Mandela was just as charming to those he didn't trust or like. Everything was a strategic move, which is why he never came away weakened or sullied by his interactions with apartheid's worst offenders.

Mandela successfully charmed, seduced and won over his apartheid jailers, the Afrikaans ideologues in Pretoria and a fearful citizenry. Mandela managed to engage without compromising his principles. While he was in jail, he learned Afrikaans and studied Afrikaans history so he could understand his oppressors. He would later use that knowledge to cajole them into compromises they didn't know they were capable of.

If politics is a game, Mandela was artful at playing it.

No doubt he would have tried to talk to and spend time with Trump so he could understand his motivations and vanities.

Mandela was tall. He stood ramrod straight and on matters of principle he would not bend.

However, when it came to tactics, he was pragmatic. I thought of Mandela when French President Emmanuel Macron rolled out a fabulous dinner at the Eiffel Tower, a military parade and the best of Paris for Trump. You wonder if Mandela would have also patted Trump on the back and called him, "My friend Donald." Maybe he would have.

Mandela was careful to never humiliate other world leaders or political opponents. That's not to say he never got angry.

Famously, Mandela couldn't hold back his dismay of the Iraq War and how the Bush administration had bypassed the United Nations: "We are really appalled by any country, whether it's a superpower or a small country, that goes outside the United Nations and attacks independent countries." He continued emphatically, "What they are introducing is chaos in international affairs and we condemn that in the strongest terms. No country must be allowed to take the law into their own hands."

Mandela firmly believed in the carefully calibrated balance of international affairs, underpinned by multilateral institutions like the UN and the sanctity of national sovereignty. I have no doubt he would have rallied against the US breaking international agreements like the Paris Accord, TPP, the Iran deal and others. He most certainly would have chaffed at Donald Trump injecting himself in the Brexit debate in the UK, for example.

Mandela was almost apoplectic when it came to abuses of power. He'd suffered under an autocratic, racist regime led by dogmatic strongmen. He felt political power was a tool, not a battering ram. He showed that by retiring after only one term, making the point that leaders have a sell-by date and that power is transitory.

As the push toward the Iraq War progressed, Mandela was furious and focused his ire on George Bush, calling him "a president who has no foresight, who cannot think properly."

Those were not empty insults, Mandela was keenly aware of the power of his moral authority. He perhaps hoped his loud clamoring against the Iraq War would change hearts, as well as minds. It didn't. Would he have been less or more successful convincing Trump on the merits of NATO or the UN?

We do know that President Trump admired Nelson Mandela -- he said so in a tweet when Mandela died. He also posted a picture of himself and Melania in what appears to be a celebrity photobomb. A frail, older Mandela looks either confused, surprised or unimpressed by the Trumps.

Trump and many others around the world admired Mandela because of his leadership. Mandela showed that a leader can be both strong and caring, uncompromising but flexible. Mandela emphasized similarities over differences. He promised peace over tribalism. And he wouldn't compromise human rights for national security.

What is clear is that leaders like Mandela and his values seem old-fashioned now: dialogue, compromise and reconciliation feel like swear words in Trump's America. Internationally, the disruption caused by the Trump administration has increased great power competition with an emphasis on a cynical, transactional politics.

President Obama acknowledged as much in his lecture to honor Mandela's legacy: "Look around, strongman politics are ascending suddenly, whereby elections and some pretense of democracies are maintained, the form of it, but those in power seek to undermine our institution and in the West you've got far-right parties that often times are based not just on platforms of protectionism and closed borders, but also on barely hidden racial nationalism."

Obama's speech -- just even talking about Mandela's leadership -- reminds us just how much has changed.

How relevant is Mandela's legacy in this increasingly zero-sum world?

And if Mandela were still here, in his prime, what would he do? – CNNs b