For instance, if you own property in Texas, Mexico and Italy, you cannot write a Texas will and assume that it will be good enough to transfer the property in the other countries too. Likewise, you should have a Texas will to bequeath your Texas property, rather than asking your heirs to try to probate a Mexican will here in Texas.
Ideally, you will have a will in each location where you own property. In fact, that may be the only way to transfer the property according to your wishes.
If you have property in other countries, make sure that each attorney that is drafting a will for you knows the whole story. For instance, you don't want to sign a new will in the United States that contains language that states that it "invalidates all other wills signed prior" because you don't want to invalidate the will that you signed last December in Mexico. However, specific language can be drafted which acknowledges your other property and other wills without a risk of invalidating them.
In some cases, a foreign will can be used to transfer property but it doesn't always work.
An attorney who is familiar with the interaction of U.S. inheritance law and inheritance law from another country can help advise you.