FIrst, as others have said, the US has never promised to fight for Israel, and nobody has ever expected it to. The most we can expect is that It will stand on the sidelines and resupply, as it did in the Yom Kippur War.
The sad truth, however, is that even if the USA had made such a promise, Israel would not be so foolish as to rely on it. The USA has a long and shameful history of abandoning its friends and allies whenever it gets tired or distracted or sees some other opportunity, or whenever some domestic political advantage can be squeezed from it, or whatever.
One of the most dishonorable episodes in US history was when South Vietnam successfully fought off a massive invasion and then turned to the USA to fulfill our solemn commitment to resupply its depleted arms and to provide air support, and the Democrats in Congress refused to fund it, KNOWING AND INTENDING that this would mean our allies would fall to the next attack and would be slaughtered and enslaved. This was unforgivable.