Jesus insisted that he came not to destroy the law, but fulfill it. He assured his followers that not one jot or tittle of the law would pass away because of him.
The Old Testament remains part of the Christian canon and continues to be revered as scripture by all Christian sects that I know of. Dismissing it as "Jesus undid all of it" is disingenuous.
The Quran and the Bible cover a great deal of the same material. Christ is a major figure in the Quran as well and is known as Isa. The principles taught by this character and the narratives surrounding him remain very similar in both accounts.
Islam is not an existential threat to the United States or the Christian world. We know this because we had peaceful relations across the Muslim world until the mid-late 1970s, 200 years into the United States's existence.
It's certainly possible that some people insist their interpretation of Islamic works mandates that they kill all non-Muslims. Similar interpretations could be made of Christian scriptures (and indeed, the Book of Joshua thoroughly documents many occasions when Israel raided cities, including occasions wherein they were instructed to kill any living person -- man, woman, or child -- that inhabited it). Nothing Jesus taught would preclude such actions by Christians today. Jesus even said that he came not to send peace, but a sword.
The important thing here is not to pretend a) that Christians are fundamentally invulnerable to the same interpretations or impulses, because we know from history that they are not; and b) that Islam is an existential threat in total based on the malicious practices or interpretations of some of its prominent sects, because we also know from history that it is not.
To the extent that it exists, the Muslim world's conflict with the West is based not on intractable doctrinal conflicts, but social and political changes that occurred in both regions/cultures over the course of the 20th century. Here's a few:
* The importance of petroleum to the West caused us to become very sensitive to the existing political powers in the Middle East
* We deposed a not-entirely-hostile democratically-elected head of state and set up a puppet government in Iran to protect our petroleum interests, which was overthrown by religious extremists.
* Muslims felt threatened by the creation of Israel
* Muslims observed the sexual revolution in the United States and really did not want that to come and disrupt their traditions
* The Sauds used the power they accumulated after the fall of the Ottoman Empire to promote a restrictive interpretation of Islam that served to entrench their dynastic monarchy, unifying religious and political authority, as is typical in dictatorships
... and so on. There's actually a lot of interesting political events that have gone into the current climate. It can't be boiled down to the simplistic and obviously false conclusion that "Islam and Christianity can't co-exist", as a) they're derived from the same traditions, such that their holy books recount many of the same narratives with only minor variations; and b) they've successfully co-existed for 1400 years, the overwhelming majority of which were not spent in conflict, and many of which were spent in cooperation and trade.
Please do not fall victim to the people trying to bait people of differing religious backgrounds against each other. That is not how peace is achieved. We need to understand each other, not belittle the others' traditions.