This thread is ten years old, but I feel like I need to add a voice of reason here for anyone referring to this in the future.
In academic settings, I have always been taught, and have always followed (without ever being corrected), the following simple and (for the most part) unambiguous conventions for writing the possessive forms of nouns:
Three Rules for Writing ALL Possessive Nouns (Common and Proper):
All Singular Possessive Nouns:
1. [singular noun] + [apostrophe] + s
Plural Possessive Nouns:
2. [plural noun whose spelling ends in "s"] + [apostrophe]
or
3. [plural noun whose spelling does not end in "s"] + [apostrophe] + s
Examples:
SINGULAR NOUNS:
dog → dog's fur
bus → bus's engine
box → box's contents
witch → witch's broom
city → city's history
man → man's name
child → child's parents
this deer → this deer's antlers
cactus → cactus's spines
crisis → crisis's origins
this species → this species's evolution
this lens → this lens's curvature
this TV → this TV's resolution
my mother-in-law → my mother-in-law's opinion
a passerby → a passerby's attention
Canada → Canada's allies
the United States → The United States's role (see examples below)
the U.S. Constitution → The U.S. Constitution's detractors
the Bill of Rights → the Bill of Rights's authors
Michelle → Michelle's sister
James → James's house
Texas → Texas's capital
Descartes → Descartes's legacy
McDonald's (the restaurant) → McDonald's's offerings
PLURAL NOUNS:
dogs → dogs' owners
buses → buses' engines
boxes → boxes' contents
witches → witches' coven
cities → cities' similarities
men → men's names
children → children's parents
these deer → these deer's habitat
cactuses (or cacti) → cactuses' spines (or cacti's spines)
crises → crises' origins
these species → these species' divergence
these lenses → this lenses' curvatures
these TVs (or these TV's) → these TVs' prices (or these TV's' prices)
their mothers-in-law → their mothers-in-law's opinions
several passersby → several passersby's attention
Some real usage examples:
"The United States's Power Scares These Seven Countries, Which See It as a National Threat"
(https://www.newsweek.com/usa-threat-countries-scared-645092)
"Can the United States’s global reputation be repaired?"
(https://www.aljazeera.com/program/inside-story/2020/11/4/can-the-united-states-global-reputation-be-repaired)
"Due to the United States’s special international status" (https://www.brookings.edu/opinions/is-america-declining/)