Queen Elizabeth II watches  a traditional performance welcoming her to Western Australia. (Mogens Johansen/AFP/Getty Images)

When Kate Middleton and Prince William reveal their child to the world, presumably before the end of the day, they will be presenting the heir to the throne of not just the United Kingdom but all 16 of the Commonwealth nations. Or that's the plan, anyway: Lawmakers have been working all year to change succession laws so that the crown will pass to this child regardless of gender.

But that's proven more difficult than they may have anticipated. Thorny politics in some of the Commonwealth states are complicating the process. Ultimately, the changes will probably go through. If they don't, though, the line of royal succession could technically bifurcate, leaving two heirs with competing claims to the throne. Or it could force one or more members of the Commonwealth – most likely Australia or Canada – to break away from the British crown. Whatever happens, what's certain is that these old, last vestiges of the British empire are once again causing headaches in London, a reminder of both how much the world has changed since the empire's collapse and the ways in which it hasn't. Managing the colonies is still a chore for the British monarch, even when the monarch is mostly powerless and those colonies are largely independent.

This all began with an old English law, predating the imperial expansion, that gives male heirs preference for the throne. This means that if Middleton and William had a girl first and then a boy, under the old law, the boy would inherit the crown over his older sister. This seeming a bit outdated, the U.K. wants to change the law so that girls have equal succession rights, as other European monarchies have done. Seems simple enough.

Here's the problem: As the British Empire expanded, it imposed not just its monarchy on far-flung colonies and possessions, but it also ingrained the rules of royal succession into the colonial constitutions. After the empire collapsed, a number of those colonies kept the British monarch but now they have their own independent governments and constitutions. And if the British royal family wants to guarantee that this baby can inherit the throne regardless of gender, it will need to convince all 16 countries in the British Commonwealth to change their constitutions, so that every one has the same succession laws. That's a lot tougher than you might think.

On the surface, this would seem like an easy process. Couldn't U.K. Prime Minister David Cameron just call up his 15 counterparts from the other Commonwealth states and ask them to amend their constitutions? He can, but there's no guarantee that every country will follow through, and it's looking possible that two of the commonwealth's biggest countries, Canada and Australia, could refuse to change their laws.

It's not that Canada and Australia think male succession is a great idea and oppose granting royal women equal access to the throne. The issue is actually that a number of Canadians and Australians want nothing to do with the British royals and think their countries should formally break away from the throne. For them – they're called republicans, as in they want to have a purely representative government with no trace of monarchy – obstructing what would otherwise be a routine constitutional amendment is a way to force a wider debate on whether Australia and Canada should remain jewels in the British crown.

The Australian and Canadian movements to break from the monarchy are far from fringe. In 2010, Australian Prime Minister Julia Gillard suggested that the country should end its place in the royal commonwealth once the current monarch, Queen Elizabeth II, dies. "I think the appropriate time for this nation to move to be a republic is when we see the monarch change," she said. Gillard is no longer in office, but her center-left Labor Party is still in power and generally favors dissolving its ties to the monarchy. For the succession rule change to pass, it must be approved by all six states in the Australian federation; any one could refuse to pass it on the grounds that they would rather break from the monarchy altogether.

Canada is tougher. Polling has generally found that about half of the country supports keeping the monarchy: The most recent poll found a bare majority of 51 percent support, up from 45 percent in 2009. Much of the opposition comes from Quebec, which after all has strong cultural links to France and is also home to a prominent separatist movement that would like to see the province split from Canada entirely. Some Canadian lawmakers, who wanted to amend their constitution to allow equal female succession but feared opposition, tried to sidestep the issue by passing a law simply asserting that British law on royal succession would define the practice in Canada. It's not clear whether or not this is constitutionally valid, though, and the Quebec Superior Court is hearing a challenge to the law's validity. Observers expect it to be ultimately appealed to Canada's Supreme Court.

Royal baby makes three — and third in line to the throne.

Click here to see the British royal succession graphic.

So what happens if Australia or Canada fail to change their royal succession laws? Let's imagine, hypothetically, that Canada's succession law goes unchanged. If Middleton has a girl and then a boy, her daughter would be the next in line for the throne in only 15 of the 16 Commonwealth states; in Canada, her son would be the royal heir. According to University College London Professor Robert Hazell, who spoke to The Guardian, this could actually divide the line of succession, establishing two different monarchs: Middleton's son would become the king of Canada, while her daughter would become the queen of the United Kingdom and all other commonwealth states that changed their laws in time.

Outlandish though this may sound, it's actually happened before. As Hazell pointed out in his Guardian interview, for a long time the British monarch also held a royal office in the German city-state of Hanover. But in 1838, the line of succession split when Queen Victoria was crowned because England and Hanover had different succession laws. If that happens again within the British Commonwealth, Hazell said, "That could lead in time to different members of the royal family succeeding in different countries."

Probably a more likely outcome, though, is that if Canada or Australia can't pass the new succession laws, then they may simply quit the British monarchy altogether. It would be, in some ways, a fitting end to one or another vestige of the once-mighty British Empire. The American colonies broke away through violent rebellion, India threw off the British crown with non-violent popular movements and South Africa stripped it away by popular referendum. Why shouldn't it be procedural politics that ends centuries of British royal rule in Canada and Australia?