Robert Peters
Since the dawn of the nuclear age, the United States has extended its “nuclear umbrella” over its closest allies. This nuclear umbrella, also referred to as an “extended deterrent,” means that the U.S. guarantees the security of a close ally—including through the use of nuclear weapons. Put simply, extended deterrence means that if an adversary carries out a strategic attack on an American ally or otherwise threatens to topple said ally through conventional means, the United States reserves the right to defend that ally with all the forces at its disposal, to include its nuclear arsenal. Generally speaking, the states that are under the American nuclear umbrella are U.S. allies in NATO, Australia, Japan, and South Korea.REF

Today, the fear of a withdrawal of America’s extended-deterrent guarantee to its closest allies is triggering actors from Poland to Korea to France to consider building their own, independent nuclear arsenals to guarantee their own security or to extend their own sovereign strategic capabilities over other allies as a means of establishing their own nuclear umbrella.REF It might seem odd that the United States extends its nuclear umbrella over other nations—seemingly, this puts the U.S. at greater risk of a nuclear attack as it defends a foreign power—with little-to-no overt benefit for the United States. But the reason for extending America’s nuclear umbrella over its closest allies is not altruism, but because doing so squarely serves U.S. national interests and makes war—including nuclear war—less likely for the U.S.

Indeed, extending the nuclear umbrella provides a variety of benefits to the United States, to include giving America greater influence over the actions of its foreign allies; reducing the incentive for other states to pursue nuclear weapons (and therefore reducing the chance that the United States will become involved in a nuclear war); and incentivizing direct foreign investment into the U.S.  » Read More
https://www.heritage.org/defense/report/extended-deterrence-tool-has-served-american-interests-1945