Isn’t it normal for Australia to help the United States with an investigation?

In general, yes. Australia and the United States regularly share information as part of the “Five Eyes” intelligence network, and government officials work together on a variety of international law enforcement issues affecting both countries, like drug smuggling, cyberhacking and copyright and intellectual property protection.

Then why is everyone making such a big deal of Trump’s latest request?

The president was trying to use American diplomacy not for subjects of shared interest, but for personal political gain. Hoping to bolster his re-election chances, Mr. Trump called for help in investigating — and ultimately undermining — the investigators who found that Russia had intervened on his behalf in the 2016 election.

Historians of American-Australian relations say they are unaware of any precedent for such a politically driven request.

What does that mean for Morrison?

In practice, Mr. Trump has asked Mr. Morrison to scrutinize his own Liberal Party colleagues as if they were potential enemies.

It all starts with Alexander Downer, Australia’s former chief diplomat in London, who helped kick off the investigation into Russian interference after a Trump adviser told him over drinks that the Russians had reported having dirt on Hillary Clinton. He passed on that information to the Americans and to Canberra, but for some, especially in the world of right-wing conspiracy theorists, Mr. Downer remains a mysterious villain.