Evidence since late 2023 shows that once additional fronts opened beyond Ukraine (notably the Gaza war and wider Middle East tensions), the U.S. did in fact face serious constraints supplying conventional munitions due to already depleted stockpiles.

1. Pre‑existing low stockpiles. Even before the Gaza war, U.S. officials and analysts described 155mm artillery and other key munitions as stretched by Ukraine, with the U.S. “struggl[ing] to match Ukraine’s immense munitions needs while not running down its own supplies to dangerously low levels.”(defenseone.com) These assessments emphasized that rebuilding stocks would take years even with expanded production.(heritage.org)

2. A second front and overlapping demands. After the October 7, 2023 Hamas attack, the U.S. began large, rapid weapons transfers to Israel, including at least tens of thousands of 155mm shells, some of which had originally been allocated for Ukraine or for U.S. reserve stockpiles in Europe.(countercurrents.org) U.S. officials explicitly acknowledged overlap in some munitions requirements for Israel and Ukraine and said Washington had to work hard to ensure both received what they needed.(frontnews.eu) This is direct evidence of constrained capacity once the second front opened.

3. Concrete supply cuts driven by low U.S. stocks. By July 2025, the U.S. government halted or paused shipments of key munitions to Ukraine—including Patriot air-defense missiles, thousands of 155mm artillery shells, GMLRS rockets, and Hellfire missiles—after an internal Pentagon review found U.S. stockpiles were “too low” to allow further transfers. Multiple mainstream outlets (Reuters, AP, Euronews, Al Jazeera, among others) report that the official rationale was concern over depleted American stocks and a decision to prioritize U.S. and other theaters’ needs.(reuters.com) Reporting on the same pause notes competing demands, especially for Patriot systems also being supplied to Israel.(aljazeera.com)

Collectively, these facts show that once an additional major conflict/front emerged, the U.S. did encounter serious constraints on providing conventional munitions because its stocks—already heavily drawn down by Ukraine—were judged too low to sustain all commitments. That matches the substance of Sacks’s prediction.