Um, you send your defenses.
Nukes are not defenses.
We call it the Department of Defense after all. Not the Department of Disproportionate Retaliation.
If the enemy is landing troops on your shores in an unwarranted war of aggression, they are saying two things:
They are willing to kill as many people as possible just so they can take a little stretch of land.
They want to annihilate your country.
At that point there is no other option but nuclear retaliation. Even JFK and Khrushchev, the guys who didn't let the Cuban Missile Crisis turn into a war, would agree with that. Even Stanislav Petrov, the Russian who ignored a false alarm and saved the world from nuclear apocalypse in 1983, would agree with that. If your home country's very existence is threatened by an enemy assault, send out the nukes.
Also the Department of Defense used to be called War Department. The current name is all marketing. We haven't used it for defense since 1941. It's all for foreign intervention and invasion. But if we did have to defend our own shores, everything that is allowed by the Geneva Convention should be options on the table.
Besides, nuking the home country thousands of miles away won't necessarily stop the invasion force, since they are, by way of being in your territory, not there.
Nuking the enemy's home country thousands of miles away will destroy their morale, supply lines, and industry, and make the invading force untenable.
When the Empire of Japan surrendered, they still had a colonial empire, and many occupation troops in China, the Philippines, and Indonesia. Their former colonies became defunct after the two nukes were dropped--American troops never directly occupied Japanese territories in China (although they did in South Korea). The troops for the most part (except for a few stragglers and rebels) surrendered to China.
Dropping nuclear bombs on Japan prevented a full-scale land invasion of the Empire of Japan, which would have led to 1 million lost American lives, 2 million lost Soviet lives, and up to 20 million lost Japanese lives.