What is US birthright citizenship over which Trump shared letter calling India 'hell-hole'?

Under the 14th Amendment to US Constitution, anyone born in US is considered a citizen at birth. In his executive order after assuming presidency, Donald Trump directed federal agencies to deny citizenship to US-born individuals if at least one parent isn't an American or a lawful permanent resident. Trump on Thursday reposted a letter calling India "hell-hole" over birthright citizenship.