Nevertheless, a sustainable rally isn'’t likely unless there' is more clarity on the US.S.'s ultimate aims, enough to ease current uncertainties. Nevertheless, a sustainable rally isn’t likely unless there is more clarity on the U.S.'s ultimate aims, enough to ease current uncertainties.

In formal writing, "U.S." is the correct abbreviation for the United States. The periods are necessary in American English to indicate that it's an abbreviation. While "there's" is often used informally, in formal writing, it's better to write "there is" for clarity and proper structure. For the possessive form of "U.S.", you should use "U.S.'s" to indicate possession. This follows standard rules for possessive forms in English when the noun is an abbreviation that ends with "S."