ISIA will attempted to implode from with in the US.. Many different war to manage a war, never has to be a full frontal attack. And all armies don't always wear a uniform or carry a flag. Covert undertaking can accomplish much.
Small, mid-size towns are a good target. Expecting the major city's like Chicago, NY, LA, etc to be hit, but places in the mid west and deeper south can be prime objectives. Less expected, the less security.
Starting vast forest fire's can be another serious weapon for any terrorist. Have 20, or so, major forest fires started at the same time would be overwhelming. Not only forest, but crops and food processing places. Our water supply would also be taxed to the max.
Speaking of water: Well placed devices around dams, water plants and other water ways bring havoc and destruction also. Hitting major natural gas line, in winter mostly, also brings extreme damage to the population.
Don't always need a invading army to win a ultimate victory. The US has a population of around 320-330 million, and most don't have a clue about the danger of open borders and any terrorist threat. ...too bad, once a great country.