Saturday marks the 100th day of U.S. President Donald Trump's term, a benchmark that has received perhaps undue attention. But the occasion can still serve as a useful milestone to review the direction of his administration's policy so far and explore the inevitable constraints that await those attempting to implement the president's vision. Newly elected presidents generally enter the White House accompanied by a political tailwind that allows them to prioritize a set of key issues and try to gain as much traction as possible before pockets of resistance are able to entrench. But Trump's political honeymoon was virtually nonexistent, and the next 100 days of his presidency point to even bigger challenges ahead....