The fast-moving developments in Trump's near-back-to-back summits with the heads of Russia, Ukraine and European powers have highlighted a pair of repeated sticking points.

Here are some key takeaways from the high-stakes meeting. President Trump on Monday said the United States will be involved in security assistance for Ukraine -- a key condition for Zelenskyy in any deal to end the war -- but did not elaborate on what exactly that would look like or give any specifics.