US has treaties with South Korea and Japan that make it it clear that the US will defend them if they are under attack. The Taiwan treaty is worded in a way that US may provide assistance or limited support in a war.
A quadrilateral nuclear disarmament treaty with Ukraine the US has signed has even more explicit wording, yet...
It is only since Xi became president, that China feels confident to have a more assertive foreign policy.
How so? There hasn't been a sharp rise in military spending, they haven't invaded anywhere, and they've largely maintained their same course on most of the disputed territories.
In comparison with China of the 90s and 00s, China of the 10s has definitely been more aggressive. In 1990s when the Philippines grounded an old navy ship on Scarborough Shoal in the south China sea to reinforce their claims, there was barely a peep from China. By the 2010s though, Chinese coast guard ships were harassing Filipino resupply ships to their outposts and China was making it's own island bases.
People think China's being more aggressive since their point of reference is the 90s and 00s, when China was frankly a pushover on the world stage. Chinese freighter gets accused of smuggling chemical weapons to Iran in 1993 and gets held hostage in the Indian ocean before nothing was found? Not even an apology from the US. Chinese embassy gets bombed by NATO forces in 1999? Nothing but words. A Chinese fighter pilot dies in a mid air collision with an American spy plane off of Hainan in 2001? No reparations for the loss of life and expensive military hardware. Anyone would think China was being more aggressive today if their baseline was that.
They engaged in combat with both Vietnam and the Philippines over islands in the 80-90's, and effectively took over Mischief Reef in 94 by building bases on an island. Not to mention what was a huge victory for them in the return of Hong Kong. That was their primary aim for much of those decades.
There probably is some growth in aggression, but nothing like "Xi's waiting for an election before invasion."
edit may have misread your post, unsure if you're comparing or detailing two steps. China has invaded uninhabited islands and built buildings to stake claims for some time. Building the uninhabited island isn't some drastic change.