(Image credit: Getty Images) Estate planning is complicated enough when both partners are U.S. citizens, but for couples where only one person is American, the rules change significantly at the border. For many affluent couples, the " unlimited marital deduction " is a cornerstone of estate planning — a guarantee that assets can pass to a spouse tax-free. However, if your spouse is not a U.S. citizen, that guarantee vanishes. Without the right structures in place, the IRS will view a noncitizen surviving spouse as a "flight risk" for taxable assets, often resulting in a massive, immediate estate tax bill. To protect your legacy and ensure your spouse’s financial security, you need a specialized tool: the Qualified Domestic Trust , or QDOT. Here, we will be focusing on noncitizen spouses who are domiciled in the U.S. It is worth noting that the rules for noncitizen spouses who aren't domiciled in the U.S. are even stricter.…