The Paris terrorist attacks Friday night left 129 dead, including one American, with 350 people injured and now the world is left wondering where the next attack will come.
As French President Francois Hollande vowed merciless revenge and British Special Forces patrol U.K. streets, U.S. cities have also increased their security measures to protect Americans from possible terrorist attacks.
Police in Washington D.C., New York, San Francisco, Chicago, Miami, Los Angeles, Boston and other major American cities have increased patrols and stepped up security efforts after the Paris terrorist attacks.
Although federal officials haven't reported any credible threats, U.S. Secretary of Homeland Security Jeh Johnson told NBC News, police were taking extra security measures out of an abundance of caution.
"At this time, we know of no specific or credible threats of an attack on the U.S. homeland of the type that occurred in Paris."
French experts, however, warn this is only the beginning and a new war on terror will see American casualties. French terrorist authority and former judge Jean-Louis Bruguiere told the New York Times, America could be a future target.
"We've taken a huge hit. Today we are unfortunately considered the principal enemy. This is only the beginning. We've entered a new phase that is going to resemble September 2001. And the U.S. will be hit."
"Obviously, one of the targets in Paris was a sports stadium. We're not taking any chances."
"Security at our games is always at a heightened state of alert."
With public spaces, tourist attractions, airports and federal building under increased security, U.S. presidential candidates are stepping up their rhetoric against ISIS and international refugees.
Republicans Donald Trump, Ben Carson, and Ted Cruz urged Obama not to accept the planned 10,000 refugees from Syria, while Jeb Bush said an effort to destroy the West is under way.
Many presidential candidates are calling for increased U.S. military action against ISIS, which has taken credit for the horrific terrorist attacks in Paris.
Until now, the U.S- led war against ISIS has done little to halt the organization from grabbing power.
The self-styled caliphate that suddenly came to power after grabbing huge swaths of territory in Syria and Iraq has been focused on seizing territory and expanding its reach, but the last two weeks have demonstrated its chilling capability.