The US has bases all over Europe, they spend a fortune on their own defensive tech, capabilities and leadership which Germany et. al. depend on, which acts as a massive deterrent, but also provides security partners with 'almost free R&D'. Also, the US specialises and practices so many things that are just far beyond the reach of pretty much all European forces.
The ability to put bases, air support of all kinds, intelligence, drones, etc. pretty much anywhere is basically beyond most of Europe. When a war broke out on their border (Libya), for better or worse there was an intervention. Obama reluctantly agreed and didn't want to play a lead role, but without US drones, AWACS, and electronic warfare, special forces on the ground, the UK and French fighters wouldn't have been able to do anything at all. This is actually quite a good demonstration of how specific capabilities are huge enablers of others.
Trump's NATO talking point about partners not spending enough is very fair, and most European nations are in violation of the treaty, which is doubly problematic because while NATO is strategically beneficial to the US, it's basically vital to European integrity -> they need it more than the US and spend only 1/2 of what was agreed upon.
If the US simply 'did not exist' - Russian Imperialism would reach just about as far as in the Soviet era. Basically, Poland would be back to being the 'buffer' between Germany and Russia.
Everyone in the 1990's thought all of that would melt with the cold war, unfortunately, neither China nor Russia moved along the expected path, so here we are.
American sanctions of Iran are not remotely big enough to make a dent in currency issues.
I do however think that as finance spreads into different areas, Americans will have more difficulty with said sanctions. Dubai exists as a financial centre basically because it's outside the direct influence or insight of American or European agencies. It's basically an entire micro-economy of dirty money.