Brazil's a rising regional power, so the United States inherent interest is to reduce its influence.
Brazil's not so much an antagonist, but the ruling party under Lula da Silva is traditionally associated with workers parties in Cuba and Venezuela. Brazil has also moved toward increased cooperation with China and Russia in defense and industrial manufacturing, as well as considered aligning its considerable influence on the oil market with OPEC, and is under constant lobbying from Venezuela to do so.
Most importantly, the United States would very much prefer that no power centers of comparable strength arise in the Western Hemisphere, and Brazil's GDP is closing in on France.
Powers in relative decline are always suspicious of rising powers, especially if they could potentially one day exert significant influence in a nearby region.
Foreign policy's a zero-sum game in the IC and defense world.
That describes why Brazil could have been a threat, if it were not so friendly, not why it should be described not-so-friendly now. Unless you want to say that any government with even the remotest socialistic roots, that trades with China and Russia more than the USA is being classified as Not-So-Friendly. While I have my doubts that it might actually be how USA administration classifies things, I hope it isn't true.
"Potential adversary" means anyone that may not acquiesce to the requests of our government, and/or form military partnerships with rival major powers.
Every nation thinks about defense in the same semi-paranoid (or "neorealist") mindset. It's a consequence of a dangerous existence in an purely anarchic system.
