Washington has repeatedly restated its "ironclad" commitment to defending South Korea, including using the "full range of its military capabilities, including nuclear".
South Korea, for its part, is eager to reassure its increasingly nervous public about the US commitment to so-called extended deterrence, in which US military assets, including nuclear weapons, serve to prevent attacks on allies.
Although the official policy of both countries towards the North -- that Kim must give up his nukes and return to the table for talks -- has not changed, experts said there had been a practical shift.
Washington has "effectively acknowledged that North Korea will never give up its nuclear programme", An Chan-il, a defector turned researcher who runs the World Institute for North Korea Studies, told AFP.
This Freedom Shield training is the first since that happened, meaning it "will be very different -- both qualitatively and quantitatively -- from previous joint exercises that took place in recent years", he added.
North Korea, which recently called for an "exponential" increase in weapons production, including tactical nukes, had been widely expected to respond with missile launches and drills of its own -- with experts saying more were likely over the course of the US-South Korean exercises.
"North Korea will use the Freedom Shield 2023 Exercise to unify its people and as an excuse to further invest in weapons of mass destruction," said Chun In-bum, a retired South Korean army general.
"More missile launches with variations in style and scope should be expected with even a nuclear test. More acts of intimidation from North Korea should not come as a surprise."
But Hong Min of the Korea Institute for National Unification said Pyongyang was not expected to "cross the red line".
The North is likely to refrain from activities "at which the US and South Korea are forced to counter strike in response", he said.
