Donald Trump hasn’t said how much territory Ukrainians will have to give to Russia in return for American “security guarantees”. Volodymyr Zelensky has compelling reasons to sacrifice as little as possible. Among them is the need to hold on to the “fortress belt”, a strategically vital 30-mile line that Vladimir Putin covets. We explain why it’s so important to both leaders. In a companion piece, we argue that even if the fighting stops, Mr Putin’s war on Western unity will continue.