WASHINGTON – United States President Joe Biden had been discussing military options since last Tuesday, when he was alerted by the Pentagon that a suspected spy balloon had entered the continental American airspace near Idaho.

By Wednesday, the balloon was hovering over Montana, and a full-blown diplomatic crisis was under way.

Military leaders advised then against shooting down the balloon, whose undercarriage was roughly the size of three buses, because of the possibility of debris harming civilians and infrastructure while it was over land.

But the arrival – and extended stay – of the balloon over American territory prompted furious calls from senior US officials to their Chinese counterparts.

It also led to the cancellation of Secretary of State Antony Blinken’s planned visit to China.

Here is a day-by-day timeline of the balloon’s flight and the decision-making in Washington.

Jan 28

The suspected spy balloon begins its controlled drift into American territory, entering Alaskan airspace near the Aleutian Islands.

At first, it appears to trackers at the US Northern Command to be just another one of China’s light probes around the edges of America’s defensive borders.

Jan 30

By the end of the day, it exits American territory and is over Canada, officials say. It carries solar panels that power its propulsion and its cameras and surveillance equipment.

Jan 31

The balloon re-enters the United States over Idaho, to the surprise of officials at the Northern Command as well as at the Pentagon.

General Mark Milley, chairman of the Joint Chiefs of Staff, alerts President Biden. The President asks for military options.

Feb 1

The balloon makes its way to the skies above Billings, Montana.

This alarms Pentagon officials as the state is home to the 341st Missile Wing at Malmstrom Air Force Base, one of three US Air Force bases that operate and maintain intercontinental ballistic missiles.

One Pentagon official describes shock at what officials view as a blatant, and poorly concealed, effort at spying. A White House official calls the move “audacious”.

Secretary of Defence Lloyd Austin, who is in the Philippines at the time, calls a meeting of senior military and defence officials to review options, per Mr Biden’s order.

Gen Milley and Mr Austin advise against shooting down the balloon while it is over land.

US officials convey to Chinese officials several times that the US military might shoot down the suspected spy balloon.

Mr Blinken tells a Chinese diplomat in Washington in the evening that the US government has the right to take any action to protect its interests.

Feb 2

Republican lawmakers and politicians begin criticising Mr Biden, after news of the balloon becomes widespread, for not taking harder action against the balloon and against China, and some demand that Mr Blinken cancel his trip.

Feb 3

The Chinese Foreign Ministry says that the machine is a civilian weather balloon that has strayed far – very far – off course and entered US airspace by accident.

But Mr Blinken says the entry of the spy balloon is a “clear violation of US sovereignty and international law”.

Mr Blinken reiterates his message to Chinese officials in a call to Mr Wang Yi, director of the Communist Party of China’s Central Foreign Affairs Commission.

Feb 4

The US Federal Aviation Administration temporarily pauses departures and arrivals at airports in Wilmington, North Carolina, and in Myrtle Beach and Charleston in South Carolina.

The agency says this is meant to “support the Department of Defence in a national security effort”.

One of two F-22 fighter jets from Langley Air Force Base fires a Sidewinder air-to-air missile, downing the balloon, which is flying at an altitude of between 18,290m and 19,810m.

By the time it is shot down off Myrtle Beach, the Chinese spy balloon has traversed the country at about 18,290m in the air, brought Americans out into their yards to squint at the sky, caused a diplomatic visit to be cancelled and opened a political debate. NYTIMES