No one knows if Trump will be able to defeat ISIS during his term. Only time will tell. It would be great if he came up with a plan to do so. It is within our capabilities to do so, given the right plan. We will see if Trump has the right plan.
I love Trump's ideas (whether he will execute these ideas well or not we will see) but you can't beat ISIS as a whole. It's an idea more than anything, with people of all races in it, all over the world. Sure you can take out a majority but there will always be evil people looking to do evil things. The only way you would "defeat" ISIS is if every world leader had every citizen interrogated and that's not possible.
When questions were directed towards Trump in the presidential campaign about the threat of ISIS, he tended to employ strongman rhetoric to get him through his answers. The vast majority of what he said - his intentions to reinstate torture, and deliberately target civilians - were illegal and unenforceable tactics. Given that he has no idea how to combat the threat of ISIS and nor does he know a great deal about the geopolitics on the ground in Iraq and Syria, it is hugely unlikely that he will be able to formulate a plan to defeat them.
ISIS is a group which feeds on fear, hate and division. Three things which Donald Trump's presidential campaign was fuelled on. To defeat groups like ISIS, people need to unite and show them that we are stronger together. If Donald Trump continues to lead with fear, ISIS will continue to thrive.
Donald Trump clearly never had a real plan to defeat ISIS. He just said whatever he thought sounded good to the masses of ignorant voters. He is horribly uninformed and ignorant about world affairs and about his soon-to-be job. He will not defeat ISIS. Even if he weren't completely ignorant of international affairs and the military, ISIS is very hard to defeat, since it is fragmented. There are members, or people who have an affinity for ISIS, all over the world. You can't defeat all of them. Trump is a fool for thinking that he can.