It was supposed to be the beginning of better relations between the US and Latin American countries - but the start of the America Summit has made the division clear.
Mexico's President Andrés Manuel López Obrador carried out his threat on Monday and stayed away from the summit after Cuba, Nicaragua and Venezuela were not invited.
He wanted to protest against the policy of “exclusion” that has been going on for “centuries”.
A US government official had previously told AFP that the US had not invited the three states to Los Angeles because of "reservations" about "the lack of democratic space and respect for human rights".
The absence of the Mexican head of state is a serious diplomatic defeat for US President Joe Biden.
Mexico is now represented by Foreign Minister Marcelo Ebrard, while López Obrador wants to meet Biden later in the summer.
Not a completely new course
Biden is trying to improve relations with countries further south.
These relationships had suffered greatly under his predecessor Donald Trump.
However, Biden has also largely followed Trump's course on trade issues, while rival China is becoming increasingly present in Latin America with investments and trade initiatives.
Biden's core concern, a regional agreement to curb migration, has also met with little approval so far.
It was only on Monday that several thousand migrants set off from southern Mexico to get to the United States.
Biden himself wants to come to Los Angeles on Wednesday.
According to the White House, a total of 23 heads of state are to arrive.
Announcements on economic cooperation, the fight against the corona pandemic and climate change are expected.