The same thing that unites the citizens of any other nation.
Yes, but Brazilian citizens cannot be extradicted to US.
If you mean the Congress of the Unites States of America, then that congress has the power to determine how non US citizens can (if they want) become US citizens. It does not have power to determine the rules for citizenship for any other country.
Till Death Unites Us was created on 2006-01-25.
US means Unites States as in USA or the United States of America
The answer will depend a lot on who you mean with "us". But in general, getting more united should be both possible and desirable.
Yes, if they are born in the US, they are US citizens.
Unites States of America
"History tells us that Europe was antagonistic to the Unites States".
The US flag symbolizes freedom and liberty that US citizens have. The US flag symbolizes freedom and liberty that US citizens have.
Yes, US citizens can travel to Australia.
Yes, US citizens can travel to Japan.
Yes, US citizens can travel to Singapore.