When referring to a specific, such as saying the US Senate, you capitalize. If you are just speaking of the senate in general, no caps. Same goes for words such as president and captain. When using these words with a specific name, or in place of a name:
Greetings, President, I hope you're well.
I caught her down by the dock, Captain.
I showed the captain to his quarters.
I visted with the president.
My favorite president is President George Washington.
'US' must be captialized, but for the 'the' you should only capitalize it if it is at the start of a sentence (Eg: The US is a big country).
No. It's not necessary.
Yes, the word "Marine" should be capitalized when referring to a member of the US Marine Corps or to anything related to the Marine Corps.
The US has 100 senators, two for each state. You need to specify which state you are referring to.
Yes.
To my knowledge, the proper capitalization is The United States of America. However, when using the letters only, it is the US or the USA.
US Border Patrol agent
That depends on whether you're using the words as regular or proper nouns. If you're referring to supreme court justices in general, without reference to a specific person or court, then there is no need to capitalize. If you're referring to a specific court, such as the Supreme Court of the United States, or the US Supreme Court, as it's known colloquially, you need to capitalize. The same rules apply to justices. If you're referring to US Supreme Court justices in general, apply capitalization only to the part of the sentence that includes proper (specifically identifying) nouns. If you're referring to a particular justice, as in, "President Obama nominated Supreme Court Justice Sonia Sotomayor in 2009," "justice" needs to be capitalized because you are using it as that particular person's title.
Yes. It is a noun referring to an organization. No, because military is a general descriptor. Take your pick.
Yes, "Fourth of July" should be capitalized as it is a proper noun referring to the date of Independence Day in the United States.
If you're referring to US History, that would be Senator Joseph McCarthy.
Yes, you should capitalize "Miss USA Beauty Pageant" as it is a proper noun and the official title of the event.