For instance, as the US, I want to place influence in Mexico. The only other influence I have in Central America is in Panama, a few spaces away.
Thematically this seems obvious, yes. They're directly connected to me. But I don't get a precise answer from the rule book.
The rules say the following [my emphasis]:
6.1.1 Influence markers are placed one at a time. However, all markers must be placed with, or adjacent to, friendly markers that were in place at the start of the phasing player’s Action Round.
Does the superpower space count as a friendly marker? According to 2.1.6, it is adjacent
2.1.6 Countries are connected to one another via the black, red and brown lines on the map. Brown lines represent connections within a region. Red dashed lines represent connections between countries in different regions. The black lines indicate connections between countries and superpowers. A country is considered adjacent to all other countries to which it is connected.
I don't see a definition of "friendly" elsewhere in the rules.