Is an alternate airport always required when flying internationally? When flying a domestic US flight, an alternate airport is not required unless the weather is below specific minimums at the destination.

What are the rules when on an international flight plan and leaving or returning to the US?

In larger airplanes, carrying the extra fuel required to fly to an alternate (which costs money) is a little silly when there isn't a cloud in the sky and there are multiple airports in the vicinity of your destination that can be easily reached with your already required 45 minute IFR reserve.  We should always have a plan for when things go wrong, but if it can be done safely, is it legal to file an international flight plan without an alternate?
As a simple anwer, no, it is not always required.
ICAO Annex 6 4.3.4.3 Destination alternate aerodromes

For a flight to be conducted in accordance with the instrument flight
rules, at least one destination alternate aerodrome shall be selected
and specified in the operational and ATS flight plans, unless:
a) the
duration of the flight and the meteorological conditions prevailing
are such that there is reasonable certainty that, at the estimated
time of arrival at the aerodrome of intended landing, and for a
reasonable period before and after such time, the approach and landing
may be made under visual meteorological conditions; or
b) the
aerodrome of intended landing is isolated and there is no suitable
destination alternate aerodrome.

Also, there are similar descriptions for the requirements for take-off and en-route alternates.
EASA adds aditional requirements in OPS 1.295 in the form of maximum flight time of 6 hours and at least two runways at the airport, but it is still allowed. I can only assume FAA does a similar thing