Israel and the United States have reached an agreement that will provide Israel an unprecedented amount of military aid over a decade. The State Department said the agreement, known as a memo of understanding, will be signed Wednesday afternoon. Jacob Nagel, Israel’s acting national security adviser, arrived in Washington on Tuesday morning to sign on behalf of his country. The agreement is expected to give Israel as much as $3.8 billion a year over 10 years, more aid than the United States has ever provided to any country. That represents a significant increase over the $3.1 billion the United States gives annually now, a figure that increases to about $3.5 billion a year with aid supplements approved by Congress. That is also much lower than the $4 billion to $5 billion a year that Israeli Prime Minister Benjamin Netanyahu sought.