Russians during US civil war
The Russian navy arrived to help dissuade Europeans from siding against the Union, and ended up saving San Francisco from a fire.

In 1863 Russia sailed a fleet of ships to New York and San Francisco. Russia was the European power the most friendly to the United States — which it valued as a rising power independent of Britain — and dispatched the fleet to US harbors in sign of friendship and support, and more significantly to help dissuade Britain and France from siding with the Confederacy.

The Russian ships stayed for several months, and during their stay a great fire broke out in San Francisco. Russian officers and sailors rushed to help put out the fire, suffering three injured, and winning the gratitude and affections of San Franciscans. (A popular story told locally says they lost six dead but historians find that unlikely.)

1863 San Francisco Fire
According to one account from 1928 the Russian intervention really saved the city:
"Suddenly the spectators began to cheer, and to cheer again and again. A thousand throats took up the cheering. The firemen were electrified when they observed boat load after boat load of Russian sailors and their officers from the Russian fleet in the harbor, landing with buckets and other fire fighting instruments... They took the places of the tired and exhausted firemen and worked hard and long at the pumps and finally conquered the fire."

1. Three of the BOGATYR's sailors were injured during the fire, one with a dislocated shoulder, another with a severely injured hand, a third reported having suffered minor injuries. The citizens of San Francisco were deeply touched by this offer of assistance, especially as some of the sailors had been injured.

A collection was taken up with Barry and Patten's saloon being the repository for the funding for making gold medals to be awarded to the injured men. David Skannell, the Chief Engineer of the San Francisco Fire Department wrote to Admiral Popov on behalf of the citizens of San Francisco expressing his gratitude for the "heroic conduct of Admiral Popoff and the officers and men under his command"

2. The city Supervisors of San Francisco issued a proclamation to the Russian Squadron which officially recognized Russian Admiral A. A. Popov, Captain Tachelisacoff, and Lieutenants Scraggin, Echren and Machov, and the Russian sailors "for the timely and efficient services so nobly rendered by them".
In the end the US stayed together as one nation, but today it may have been better for Russia if America was a weaker, balkanized nation. But if you're looking for a US president guilty of "collusion" with the Russians, look no further than "Honest Abe" Lincoln, the father of the modern USA.