WASHINGTON — President Obama warned Syria on Monday that it would face American military intervention if there were signs that its arsenal of unconventional weapons was being moved or prepared for use. It was Mr. Obama’s first direct threat of force against Syria, as he has resisted being drawn into the bloody 18-month rebellion. [...] “We cannot have a situation in which chemical or biological weapons are falling into the hands of the wrong people,” Mr. Obama said in response to questions at an impromptu news conference at the White House. “We have been very clear to the Assad regime...